To first answer this question, let’s take a look at the circumstances of their service history.

By the time of Operation Barbarossa in the Summer of 1941, the KV-1 had been in Red Army service for almost two years and was already receiving rather overkill armor upgrades. For example, the KV-1E M1940 production series had additional 20mm steel plates bolted onto the hull and turret to increase the effective frontal and side protection to over 95mm in effective thickness, which is not taking into account for any sloping either. The KV-1 was immune to every single German anti-tank gun of the time of the invasion, with the exception of 88mm FlaK guns and occasionally point-blank (within 100 meters) shots to the side from 50mm PaK 38’s (this was extremely rare).

Knocked KV-1E

The T-34, assuming we are referring to the M1941 variant, was a slightly different animal. Its frontal plate worked equally well in protecting against German AT guns, just as the KV-1 did, but due to the very strong angle of the frontal plate, the T-34 relied more on ricochets to stop rounds rather than actual armor thickness. As a result, HEAT ammunition (typically fired from German 75mm howitzers) were capable of knocking out T-34’s, while they did not prove as useful against KV-1’s. The sides of the T-34 were more vulnerable too, and they could be penetrated by most contemporary anti-tank weapons within 500 meters. However, I do mean the armor behind the suspension system, which was not sloped like the armor above it.

T-34–57

The turret wasn’t great either. It was a very awkward design and relied much more on its small size for protection than it did on armor. There wasn’t much of a slope to observe on the turret and 50mm and 75mm rounds could go through it fine. Most of the time.