unwelcome guest vs trespasser

unwelcome guest and trespasser both are nouns.

Word	Noun	Adjective	Verb	Adverb
unwelcome guest	Yes	No	No	No
trespasser	Yes	No	No	No

As nouns, trespasser is a hypernym of unwelcome guest; that is, trespasser is a word with a broader meaning than unwelcome guest:

unwelcome guest: someone who gets in (to a party) without an invitation or without paying
trespasser: someone who intrudes on the privacy or property of another without permission

Other hypernyms of unwelcome guest include interloper, intruder.

unwelcome guest (noun)	trespasser (noun)
someone who gets in (to a party) without an invitation or without paying	someone who intrudes on the privacy or property of another without permission