harvest moon vs full moon

harvest moon and full moon both are nouns.

Word	Noun	Adjective	Verb	Adverb
harvest moon	Yes	No	No	No
full moon	Yes	No	No	No

As nouns, full moon is a hypernym of harvest moon; that is, full moon is a word with a broader meaning than harvest moon:

Other hypernyms of harvest moon include full-of-the-moon, full, full phase of the moon.

harvest moon (noun)	full moon (noun)
the full moon nearest the September equinox	the time when the Moon is fully illuminated