life buoy vs life preserver

life buoy and life preserver both are nouns.

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

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

life buoy: 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 life buoy include flotation device, preserver.

life buoy (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