Sven is a very viable competitive champ because of his ability to cc and put out high damage. Sven LOVES raw stats and needs strength in his item build. He suffers from low mobility and severe early game mana issues, but if he is built correctly he can excel and go beyond his negative aspects. He is an initiator and fairly tanky.
1Q. Storm Hammer - This skill is Sven's bread and butter. This should be maxed right away as it will CC and throw out early devastating damage. Note that it is an AoE stun and will effect all enemies in its radius so use it strategically.
2W. Great Cleave - A useful skill later on, it essentially gives Sven the ability to cleave all enemies near the edge of his bladed strikes. Damage goes up as it is leveled, leave this spell for last.
3E. Warcry - A great team fighting ability that increases armor as well as movement speed for you and surrounding team heroes. This a great ability to pop in team fights and for chasing down speedy enemies.
4R. God's Strength - This skill pairs lovely with the items I have given you. Yes it simply increases melee attack power; however, when used with Armlet and Black King Bar this skill becomes a power house of vicious swings.
The most important thing to note here is maxing Storm Hammer while getting Stats as it will give you more mana to throw out more Storm Hammer's and up your damage, survivability and overall effectiveness.
Note that Warcry and Great Cleave can be used as a preference; however, what you should take away is that you should max Storm Hammer ASAP and get Stats up until level 8 or so. Always grab God's Strength whenever possible. Great Cleave and Warcry can be picked up later as a preference either or, I like to get at least 1 point in Warcry to help with chasing and escaping on a team scale. I recommend grabbing one point in it at level 8 and at least before level 10.
Armlet - Great for several reasons. First off it gives you some raw strength as well as a bit of regen, faster attacks, and armor. The active ability allows you to deal huge amounts of damage in synergy with Sven's ultimate.
Bracer - Simple, raw stats.
Magic Wand - Sven's mana issues can be easily solved with this item. A cheap little item that works wonders.
Power Treads - Although I put Phase Boots as a situational item I HIGHLY recommend Power Treads over any boots, always. The strength is Sven's best friend.
Blink Dagger - This is a great item to jump in and activate your melee abilities ( God's Strength, Black King Bar & Armlet) after stunning or prior to stunning the escapees.
Black King Bar - Another amazing active ability to synchronize with Armlet and God's Strength, also will save you from a CC heavy team.
As Sven you are going to want to be aggressive and make sure you are getting the last hits, farming is important because you need Armlet and Blink Dagger as quickly as possible. The reasoning behind the consumables is to make sure you stay in lane long enough to grab either Helm of Iron Will and/or Magic Wand for survivability. If you are being harassed hard or soloing don't be afraid to use Storm Hammer on low health creep groups to farm if you are low health and pushed back. If you have a jungle and you are soloing, keep your distance and communicate with the jungle for effective ganks using your Storm Hammer. As Sven you are the glue of the gank and will deal a good amount of damage with a stun long enough for you and your teammate to jump on the opponent and eliminate him/her.