This is my first guide so constructive criticism is welcome, but if you're just going to be an *******, don't bother.
Okay, first things first. Sven is an awesome hero and not as hard to play as some of the other carry heroes. He is crazy strong middle to late game and counters illusion heroes and physical damage heroes hard. He is vulnerable however to kiting and disables but with the right build, that can be countered.
Your goal here is to get as many last hits and denies as possible. If you can get a kill, great! But focus on your farm. You want to get your mask of madness as soon as you can to start clearing the jungle. When you have enough money to buy your morbid mask, start auto-attacking and clearing your creep waves as fast as possible so you can go farm jungle.
This is important:
Don't jungle permanently! The jungle is the nice desert at the end of your meal, it is not your main food item! Clear your creeps, jungle until there is a new creepwave near, then go kill that creepwave and jungle again. Stack whenever you can. Ask supports and teammates to stack camps every time they can. Especially ancients. With level 6 ultimate and mask of madness you can easily clear ancient stacks with warcry.
Once you have echo sabre on top of mask of madness you're pretty strong (if you get it early) and you can participate in teamfights if necessary, however, I suggest waiting until you have blink dagger as well.
Once you have blink dagger, you push and defend with your team. Try to wait until a fight has been initiated and then Ulti, blink in and stun, then activate mask of madness and wreck **** sideways. Before you have Black King Bar, try not to initiate teamfights yourself as Sven is only useful when he is hitting people. This being said, if you see an enemy hero that you think you can take and he is alone, massacre him and hear the lamentations of his teammates.
Always target disablers first. You want the Lions and Banes of this world dead as soon as possible before your BKB runs out. This minimizes the risk of them kiting you. Sven can easily manfight most enemy carries if they don't have disablers with them. That being said, troll warlord and medusa and the like will ruin your day if you don't get the jump on them, and maybe even then you should be careful if they have similar farm to you.
Your goal as Sven is not epic long teamfights. You want people dead. Fast. You want to burst people down before they can fight back. If you get a good initiation and storm bolt 2 or 3 heroes, you can easily kill them before their teammates can react. Thats why you bob and weave. Blink in, stun and wail on someone, then when their teammates show up, back off and wait to initiate again. Rinse, repeat and rampage.
Her stun, slow and mana aura allow you to chase heroes better and spam your stun more.
Reverse polarity and then Sven blinks in and cleaves their team into oblivion. Done and done. His empower ability increases your attack damage and cleave as well.
His heals and magic immunity are invaluable on Sven. Early blink, mask and echo together with repel makes Sven borderline OP in early teamfights.
Dazzle's ultimae, weave (which lowers enemy armor while increasing allied armor) together with the massive physical damage that Sven deals, is a recipe for pure destruction. Shallow grave protects Sven or allows him to deal more damage before dying. The slow of poison touch is also invaluable as a Sven ally, as Sven is not the greatest chaser.
There are many others, these are just some off the top of my head
Brown boots - We skip power treads to get an earlier boots of travel. BOT's have a 50 ms advantage over power treads and open up an item slot. Sven runs out of item slots fast!
Mask of madness - attack speed and lifesteal. This enables Sven to farm insanely fast and wreck teamfights. Always activate this after you've activated your stun, ulti and warcry! Never before. Also useful when chasing together with warcry
Echo sabre - We choose this over Sange and Yasha as it is much cheaper, solves Sven's mana problems and that double strike at the start of Sven's initiation is sick. I pray to Osfrog that they make echo sabre disassemble-able. You really need the intelligence that it gives you. Then you can just disassemble echo sabre late game to make orchid/bloodthorn and just sell the ogre axe. A man can but hope and dream.
Blink dagger - Sven needs an initiation. He is not very useful without it. This enables you to wreck teams. Running in and hoping you reach the enemy before they start kiting you doesn't cut it. Another option is shadow blade into silveredge. Particularly if the enemy team has a bristleback or troll warlord. Basically any heroes with pesky passive abilities(including huskar and viper).
Black King Bar - Once Sven has jumped into the fight, he is priority number 1 on the enemy teams's list of people to stun or disable. This is why you buy Black King Bar. With it you destroy teams. Without it, you lose your opportunity to massacre. You might quite well die without it. Note: There are abilities that counter this. Duel. Bane's ultimate. Batrider's ultimate. Axe's berserker's call. More on this later.
Crystalis - Sven loves crits. Be they Daedalus or Bloodthorn. You need one of them! You can easily crit their entire team to death under 4 seconds with God strength, crits and attack speed items. Your choice depends on the enemy team heroes. Bloodthorn if you need a silence or true strike (ie. against storm spirit, windranger, phantom assassin and the like), Daedalus for everyone else.
Assault cuirass - Sven loves attack speed. He craves it. The extra armor is awesome. Get it against high physical damage heroes. Together with war cry, most physical damage carries can't touch you. Try killing 5k hp Sven with 50 armor (normal items with satanic and AC).
Butterfly - Evasion, attack speed, evasion and agility. This equals more damage and armor and better chasing(use active to run faster), together with a middle to finger to right click carries without monkey king bar or bloodthorn
Satanic - The extra strength and damage this gives Sven makes him a monster. It's active can also save your *** if you can still hit people. Super low health? Activate satanic, get 1 crit. Boom. Hp for days. I consider this a survivability item more than a damage item.
Skadi - I don't really like Skadi on Sven. I would only get it against heroes like medusa and fast heroes who can kite you (Skadi slows through magic immunity). The stats are great but you would rather be bursting someone. Get this for long teamfighting.
Linken's sphere - against heores with single target diables abilitites that ruin your day. Buy against Doom, Batrider, terrorblade, legion commander etc.
Sange and Yasha - Most guides recommend this item, and it situationally very good on Sven. But I feel it's superfluous and too expensive for what it gives you. Power treads and SnY give Sven 438 movement speed and costs 5750 gold. Boots of travel and echo sabre give you 429 movement speed and costs 5150 gold. The SnY option gives you marginally better stats, but echo sabre allows you to kill enemies quicker and opens up an item slot. SnY is not a late game item anyway. When you sell it you lose about 2100 gold and 700 gold for selling power treads to buy BoT's. You lose only 1300 gold when you sell echo sabre. Echo sabre just makes more sense to me. But everyone has their own opinion.
Silver edge - counters bristleback, troll warlord, huskar, phantom assassin, echantress and spectre.
Blademail - cheap item if you are behind and getting focused by high damage carries enemy heroes who are squishy. Huskar, PA, Lina, Drow, Sniper etc.
Heaven's halberd - counters right click carries like ursa, clinkz, medusa etc.
Manta style - counters heroes with silences. Clinz, storm spirit etc. If you use manta while under God strength, your illusions do a ****load of damage!
Monkey king bar - buy against heroes with evasion. Phantom assassin, Windranger, brewmaster, anyone who buys butterfly or heaven's halberd.
That's all folks! Hope the guide helps you and allows you to massacre to your heart's content!