There is only one file type that uses the .KV6 extension:
Ken's Voxlap 6 Model File
A KV6 file is a Ken Silverman's version 6 model file which is also known as Ken's Voxlap 6 (KV6) and used by games with the Voxlap engine. It contains model and texture information for game objects, such as weapons and body parts.
Ken Silverman is a game programmer that wrote the Build engine in the 1990s, which was used in a variety of games, most notably Duke Nukem 3D. He began development on the Voxlap engine in 2000 and released the engine source code in 2005. The Voxlap name comes from "voxel," which is the combination of "volume" and "pixel" (or "element") and is a value on a grid in a 3D space or as Ken puts it, "is like a 2D pixel, but in 3D - just a fancy name for a cube." The Voxlap engine was used by the development version of Ace of Spades, an action game that is a blend between Minecraft and a first-person shooter (FPS). However, in 2012 the game moved to an OpenGL-based engine. You can use Slab6 to edit the KV6 file.
Updated: October 4, 2016