it is an acronym for Kliment Voroshilov, which were a series of heavy tanks created by the Soviet Union during WW2. Several of these tanks are used in the game World of Tanks. It's powerful but extremely slow.
From the Online multi-playergame, World of Tanks, it refers to the biggest and most explosive slowly reloading gun that one can have for a particular tank.
Hey, do you have derp on that KV tank? No, I have the stock gun. Noob.