Here is a description of KGB taken from the TF2 blog:

“The unlockable for the Heaviest pugilists out there is the K.G.B. Any time a Heavy kills an enemy with the K.G.B he receives five seconds of guaranteed critical hits. This crit-boost can be used to punch and kill further enemies, extending the boost time. Alternatively, the Heavy can switch to another weapon and use the boost to take out an enemy at range. The Heavy’s slow movement speed, and the K.G.B’s slower swing speed, make it a risky prospect.”

The achievements themselves haven’t yet been revealed, but you can still take a look at the beautiful icons and try to guess what they mean! My favourite so far is “Icing on the Cake.” My guess is that it probably has to do with killing a spy somehow.

The TF2 Blog + downloadable icons

Update: The descriptions of how the achievements can be earned is now on the site