From Wikipedia: As the war continued, the KV-1 continued to get more armor to compensate for the increasing effectiveness of German weapons. This culminated in the KV-1 model 1942 (German designation KV-1C), which had very heavy armor, but lacked a corresponding improvement to the engine.
Tankers complained that although they were well-protected, their mobility was poor and they had no firepower advantage over the T-34 medium tank.
In response to criticisms, the lighter KV-1S was released, with thinner armour and a smaller, lower turret in order to reclaim some speed. Importantly, the KV-1S also had a commander's cupola with all-around vision blocks, a first for a Soviet heavy tank.
However, the thinning-out of the armor called into question why the tank was being produced at all, when the T-34 could seemingly do everything the KV could do and much more cheaply. The Soviet heavy tank program was close to cancellation in mid-1943.
In Flames of War: