Multiple meanings based on context and site usage: Generally, a term for an unofficial, themed, music videos made for a song. Similar to the "A" in "AMV" (Anime/Animated Music Video), the "P" in "PMV" describes the medium or designated theme for the music video.
Currently, a PMV may be a:
Picture Music Video - uses still images in a slideshow format or stylized lyrics to accompany the music;
Pony Music Video - made with scenes from the "My Little Pony" series;
Porn(ographic) Music Video - utilizing visuals and audio from various adult resources;

A term used to reference cannabis/marijuana/weed.

Porn Music Video

PMV is an internet slang abbreviation for Picture Music Video. A Picture Music Video is a video similar to an Animated Music Video AMV. However, PMV's are created by using still drawings to portray a story. Some Picture Music Videos contain lyrics to a song used in the video. Picture Music Videos can be found on the popular video website Youtube. People usually make PMV's containing characters from their favorite t.v shows, books, movies, and video games. Some artists will even use characters of their own.

Pony Music Video. Similar to AMV (Anime Music Video) but using footage from the cartoon My Little Pony: Friendship is Magic. Due to explosive popularity of the cartoon in some internet communities and the fact that most episodes are available on Youtube free of charge, many fan made videos were created, and the term PMV is sometimes used to denote these videos. Some of the more well known ones are synced with the audio from trailers for Starcraft 2, Inception and other popular media.

A commonly used acronym for a Porn Music Video. Having it's roots deeply entrenched in Anime Music Videos(AMVs) this version differs in that you use videos of people with their roots deeply entrenched in other people instead.

PMV is an internet slang abbreviation for Picture Music Video. A Picture Music Video is a video similar to an Animated Music Video AMV. However, PMV's are created by using still drawings to portray a story. Some Picture Music Videos contain lyrics to a song used in the video. Picture Music Videos can be found on the popular video website Youtube. People usually make PMV's containing characters from their favorite t.v shows, books, movies, and video games. Some artists will even use characters of their own.