pun vs fun

pun and fun both are nouns.

Word	Noun	Adjective	Verb	Adverb
pun	Yes	No	Yes	No
fun	Yes	No	No	No

As nouns, fun is a hypernym of pun; that is, fun is a word with a broader meaning than pun:

pun: a humorous play on words
fun: verbal wit or mockery (often at another's expense but not to be taken seriously)

Other hypernyms of pun include play, sport.

pun (noun)	fun (noun)
a humorous play on words	activities that are enjoyable or amusing
	violent and excited activity
	a disposition to find (or make) causes for amusement
	verbal wit or mockery (often at another's expense but not to be taken seriously)

pun (verb)	fun (verb)
make a play on words