I have briefly visited this topic for 122 mm guns specifically, but since Wargaming is adding the S-34 into the mix, I’ll start from the top.
The KV-1 was king of the battlefield, enjoying comfort that the “Queen of the Desert” could only wish for. Its powerful F-32 gun, and later ZiS-5, was capable of handling any armour the enemy could throw at it. Later on, as high velocity 75 mm guns became more common and 88 mm guns became more mobile, the armour of the tank became more of a liability than an advantage. The KV-1S shaved off some of this unnecessary armour, leaving the tank largely invincible to 50 mm guns, but making it faster and more reliable. The gun remained the same. In 1942, the ZiS-5 was enough to handle nearly any threat. The ballistically identical F-34 enjoyed the same privileges. The gun was good enough, to the point that it was the “original” gun of the first IS tank (Object 233IS).
In 1943, Tigers and Panthers started roaming the battlefield, and the 76 mm guns weren’t quite so impressive anymore. However, 85 mm guns already proved themselves worthy against these new threats. With the SU-85 already on the battlefield, IS-85 starting production, and T-34-85 already in development, why not put the good old 85 mm gun on the KV-1S? Done and done! The result was named KV-85. The first KV-85 was just an IS turret on top of a KV-1S hull. The second had a slightly enlarged turret ring and turret platform, for more comfortable operation.
When initial parts shortages stopped constricting IS-85 production, the KV-85 became obsolete. But why waste a perfectly good tank? The KV-1S already proved capable of receiving IS turrets, so it was possible to put an IS-122 turret on there. Enter, the KV-122!
However, since the whole point of the KV-85 was to serve as a buffer while IS-85 production got off the ground, there was not much of a point in the KV-122. Only one was ever built. By the way, note the muzzle brake is the late “domestic” version, not the early “German” version like in the game. This is a bona fide D-25T, not the nerfed D-2-5T. I will not discuss the KV-122 in great detail, since I already did so in this article.
If you have played the KV-1S, odds are you are familiar with these guns. However, SerB announced that, regardless of any other changes, the KV-1S will receive the S-34 gun. The what now? Let’s find out!
Grabin’s construction bureau, the one that tried to put a 107 mm gun into the IS-4 (not the Object 701, the other one), kept on going for gold, despite losing out on the first two IS tanks. This time, they hedged their bets on the 100 mm caliber. Two IS prototypes received 100 mm guns: the IS-4* (D-10) and IS-5 (S-34). The S-34 gun had all sorts of cool stuff, like a loading assist and stabilization mechanisms. You can read more about the gun and the IS tanks that carried it here. The KV isn’t mentioned anywhere, but since sticking IS turrets on KV hulls was basically second nature for the Red Army by that point, it was only a matter of time until this thing showed up.
The gun was equivalent to the D-10 in penetration (expect the same 175 mm), but had superior ergonomics, rate of fire, and accuracy. If it retains all these qualities in game, it will be a worthy successor to the D-2-5T.
Edit: I have been reminded of another project, a KV-1S (not even a KV-85) with a 122 mm howitzer.
The goal of the project was to combine the agility of the T-34 and firepower of the SU-122. The result was pretty impressive: the gun was capable of 6 aimed shots per minute, compared to the SU-122′s 5 shots. This was achieved by saving space inside the turret due to the smaller recoil mechanisms, compensated by the muzzle brake.