The extension M4V denotes MP4 files containing only video streams.
The MPEG-4 Part 14 or MP4 format, which is marked by the extension mp4 as avi and mov a container format. It is based on Apple Quicktime.
The video streams may be encoded in various compression methods.
Supported MPEG 1, 2 and 4. MPEG 4 contains such codecs as Xvid, Divx and H264 and is mostly used.
mp4 files can also contain additional metadata.
The contents of the file may also be protected by DRM (Digital Rights Management). Then you need a corresponding, possibly paid license to view the content.
For mp4 files that contain only one type of data, other endings are also in use. M4a for audio, m4b for audiobooks.