navy vs military service

navy and military service both are nouns.

Word	Noun	Adjective	Verb	Adverb
navy	Yes	No	No	No
military service	Yes	No	No	No

As nouns, military service is a hypernym of navy; that is, military service is a word with a broader meaning than navy:

navy: an organization of military vessels belonging to a country and available for sea warfare
military service: a force that is a branch of the armed forces

Other hypernyms of navy include armed service, service.

navy (noun)	military service (noun)
a dark shade of blue	land tenure by service in the lord's army
an organization of military vessels belonging to a country and available for sea warfare	a force that is a branch of the armed forces