FLV : Flash Video is a container file format used to deliver digital video content (e.g., TV shows, movies, etc.) over the Internet using Adobe Flash Player
MKV : The Matroska Multimedia Container is an open standard, free container format, a file format that can hold an unlimited number of video, audio, picture, or subtitle tracks in one file.
MP4 : MPEG-4 Part 14 or MP4 is a digital multimedia container format most commonly used to store video and audio, but it can also be used to store other data such as subtitles and still images.