The Kliment Voroshilov (KV) tanks were a series of Soviet Red Army 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 part of World War II, especially during the first year of the invasion of the Soviet Union.
Almost completely immune to the 3.7 cm KwK 36 and howitzer-like, short barreled 7.5 cm KwK 37 guns mounted respectively on the early Panzer III and Panzer IV tanks fielded by the invading Nazi forces; until better guns were developed by the Germans it was often the case that the only way to defeat a KV was with a point-blank shot to the rear.
Prior to the invasion, about 500 of the over 22,000 tanks then in Soviet service were of the KV-1 type. When the KV-1 appeared, it outclassed the French Char B1, the only other heavy tank in operational service 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 became a base of development for the Joseph Stalin (Iosif Stalin, or IS) series of tanks.