eternity vs existence

eternity and existence both are nouns.

Word	Noun	Adjective	Verb	Adverb
eternity	Yes	No	No	No
existence	Yes	No	No	No

As nouns, existence is a hypernym of eternity; that is, existence is a word with a broader meaning than eternity:

eternity: a state of eternal existence believed in some religions to characterize the afterlife
existence: the state or fact of existing

Other hypernyms of eternity include being, beingness, face of the earth.

eternity (noun)	existence (noun)
a state of eternal existence believed in some religions to characterize the afterlife	everything that exists anywhere
time without end	the state or fact of existing
a seemingly endless time interval (waiting)