Killing Floor Mod
The original Killing Floor only has two modes by default: Killing Floor (also known as Survival) and Objective Mode. However, these two modes can be furthered refined by using Sandbox Mode or Mutators.
Additionally, if you own the free Toy Master addon, you have access to a third mode simply called Toy Master.
Lastly, content creators are able to make their own custom modes, such as Gun Game or Vehicle Mod.
There are five difficulty levels available for each map. Additionally, for Survival Mode you can specify a Game Length of Short (4 Waves + Boss), Medium (7 Waves + Boss), Long (10 Waves + Boss), or Custom. Some achievements can only be unlocked by playing a Long game.
Killing Floor 2
Lastly it has Weekly Outbreaks which modify a standard Survival match with special rules.
There is no Beginner difficulty in this game, so there are only four difficulty choices per map. Game Length options remain the same as the original game.
Killing Floor: Incursion
Additionally there are only two difficulty levels: Normal and Hardcore.