M2V is short for MPEG-2 Packetized Elementary Stream. It is the MPEG-2 video stream file.
MPEG-2 video streams are usually stripped from the audio so they can be multiplexed (muxed) with new audio and perhaps subtitles before being put into one new transport stream.
An M2V stream is created by using a demultiplexing (demuxing) software from an existing MPEG-2 transport stream.
See also MPV