There is only one file type that uses the .MPV extension:
MPEG Elementary Stream Video File
Video file containing an "elementary stream," or raw video data; typically contains video data stored with MPEG-2 compression; used for raw output from a video recorder or for raw input to a video authoring program; does not contain audio data.
MPV files do not use a container format like .AVI files or .MP4 files. However, they may still be played in common video players that have the appropriate codec. Sometimes an MPV file can be renamed as an .MPG file, which may allow it to be played in common video players.
MPV files are similar to .M2V files.
Updated: July 3, 2015