cover letter vs missive

cover letter and missive both are nouns.

Word	Noun	Adjective	Verb	Adverb
cover letter	Yes	No	No	No
missive	Yes	No	No	No

As nouns, missive is a hypernym of cover letter; that is, missive is a word with a broader meaning than cover letter:

cover letter: a letter sent along with other documents to provide additional information
missive: a written message addressed to a person or organization

Other hypernyms of cover letter include letter.

cover letter (noun)	missive (noun)
a letter sent along with other documents to provide additional information	a written message addressed to a person or organization