Fast and hurting, these can create a havoc in battlefield. They are fast and have decent damage, but disabled crtitical hits. Uber/Super knife, have retrived crtical hit chance, well as bigger damage, but they are slower. Royal knife/Konrad's Knife is the knife you start equiped with.
Machete's have splash damage, well as litlle slower swings. Also, Enemy needs to be pointed with crosshair, in order to deal damage. Exclusive golden versions were obtainible by getting 700 kills, in event of Halloween 2011.
Slow, and dealing a small damage, unless the target is 1 meter before you. But, deals damage on 5 meters, proving that, they have longer range, and possibly, can hit two enemies at once. Well, these are not used amd disliked by community.
Close quarters melee weapon for those silent moments.
The knife is the game's only melee weapon. For a short while, Nikolai's Nocturnal and Grim's Varg sets had an attached melee ability, but it did only light damage and couldn't deal critical hits. It made for very amusing Melee-only fights in the PTE though, where it was readily available to everyone for a few VP.
Incidentally, many people consider the Nationals to have more knifers than the Royals do. Perhaps the smaller frame is harder to spot when you're not stealthed.