The Kliment Voroshilov (KV) tanks were a series of Soviet heavy tanks named after the Soviet defense commissar and politician Kliment Voroshilov and used by the Red Army during World War II. The KV series were known for their extremely heavy armour protection during the early part of the war, especially during the first year of the German invasion of the Soviet Union.
They were practically 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 German forces. Until more effective guns were developed by the Germans, the KV-1 was invulnerable to almost any German weapon except the 8.8 cm Flak gun. Even then, the sloped and reinforced front armor and turret of the KV-1 caused some hits from the 8.8 cm Flak gun to ricochet.
Prior to Operation Barbarossa (the German invasion of the USSR), 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 well) in all practical respects. Later in the war, the KV series became a base for the development of the IS (IS - Josif Stalin) series of tanks.