In the U.S., we don't use "canteen" in this sense. It sounds military to me. Perhaps it is used in the armed forces. I don't know.

No difference whatsoever, a canteen and a cafeteria are synonymous. Although canteen can also mean a flask or container for water, however, I imagine that would be obvious from the context.

Canteen was usually used for a ‘works canteen’ - the cafeteria which served a factory or industrial site, and also in the armed services in the same way. In most other places ‘cafeteria’ is more commonly used.