fountain vs hot spring

fountain and hot spring both are nouns.

Word	Noun	Adjective	Verb	Adverb
fountain	Yes	No	No	No
hot spring	Yes	No	No	No

As nouns, hot spring is a hyponym of fountain; that is, hot spring is a word with a more specific, narrower meaning than fountain:

Other hyponyms of fountain include Fountain of Youth, geyser, thermal spring.

fountain (noun)	hot spring (noun)
a plumbing fixture that provides a flow of water	a natural spring of water at a temperature of 70 F or above
an artificially produced flow of water
a structure from which an artificially produced jet of water arises
a natural flow of ground water