The Kliment Voroshilov (KV) tanks were a series of Soviet heavy tanks, named after the Soviet defense commissar and politician Kliment Voroshilov. The KV series were known for their extremely heavy armour protection during the early war, especially during the first year of the invasion of the Soviet Union in World War II. Almost completely immune to the 7.5 cm KwK 37 and 3.7 cm KwK 36 guns mounted on the early Panzer III and Panzer IV tanks, until better guns were developed often the only way to defeat a KV was a point-blank shot to the rear. Prior to the invasion, about 500 of the over 22,000 tanks in Soviet service at the time were of the KV-1 type. When the KV-1 appeared, it outclassed the French Char B1, the only heavy tank used in the world at that time. Yet in the end it turned out that there was little sense in producing the expensive KV tanks, as the T-34 medium tank performed better (or at least equally) in all practical respects. Later in the war, the KV series have become a base of development of excellent Iosif Stalin tanks.
With thick armour for its tier and guns that can seriously harm anything up to tier VIII, this is a great heavy tank. Unfortunately the stock speed and maneuverability are amongst the lowest in the game. In addition, the upgraded turret which is required for the high tier guns has a very slow traverse speed. Unfortunately the KV is a huge target, and so slow that it is easily hit by artillery.
As all of the good guns for the KV (122mm, 152mm and 107mm) have low accuracy and a low rate of fire, getting close to the enemy is key to success. Try to use buildings and other cover to your advantage, as only higher tier tank destroyers and artillery are serious threats to you.
Get a good driver and the upgraded engine before getting better guns. The 107mm is by far the best weapon for this tank, but the 122 and 152mm guns can be very powerful at close range and their high damage makes them often one-shot tanks when fighting lower or equal tier enemies.