prison guard vs keeper

prison guard and keeper both are nouns.

Word	Noun	Adjective	Verb	Adverb
prison guard	Yes	No	No	No
keeper	Yes	No	No	No

As nouns, keeper is a hypernym of prison guard; that is, keeper is a word with a broader meaning than prison guard:

Other hypernyms of prison guard include law officer, lawman, peace officer.

prison guard (noun)	keeper (noun)
someone who guards prisoners	one having charge of buildings or grounds or animals
	someone in charge of other people