lifesaver vs life preserver

lifesaver and life preserver both are nouns.

Word	Noun	Adjective	Verb	Adverb
lifesaver	Yes	No	No	No
life preserver	Yes	No	No	No

As nouns, life preserver is a hypernym of lifesaver; that is, life preserver is a word with a broader meaning than lifesaver:

lifesaver: a life preserver in the form of a ring of buoyant material
life preserver: rescue equipment consisting of a buoyant belt or jacket to keep a person from drowning

Other hypernyms of lifesaver include flotation device, preserver.

lifesaver (noun)	life preserver (noun)
a life preserver in the form of a ring of buoyant material	rescue equipment consisting of a buoyant belt or jacket to keep a person from drowning
an attendant employed at a beach or pool to protect swimmers from accidents