That is was composed by Johann Pachelbel and is called the Canon in D.
Pachelbels Canon in D is not Christmas music. Perhaps someone has combined this piece with something else, but it wouldn't have been composed by Pachelbel.
One is the Concerto for Two Violins in D Minor by Bach. It is a duet that sounds pretty good.
Pachelbel's Or Canon in D, by Pachelbel.
There are reasonably good keyboard versions of the Canon in D.
Look up "Canon in D major for violins" in a serach engine.
Pachelbel's Canon (Canon in D Major by Johann Pachelbel)
The chords for "Canon in D" by Pachelbel are D major, A major, B minor, F minor, G major, and G minor.
The chords for Pachelbel's Canon in D are D major, A major, B minor, F minor, G major, and G minor.
Canon in D is Pachelbel's most famous composition. It was composed in early 1600's. It is paired with a gigue in the same key.
Some musical bands that incorporated the melody of Pachelbel's Canon in D in their songs include "The Pop Tops" and "Aphrodite's Child." You can learn more about Pachelbel's Canon in D at the Wikipedia.
Canon in D
The tone or key would be D.