belly vs tummy

belly and tummy both are nouns.

Word	Noun	Adjective	Verb	Adverb
belly	Yes	No	Yes	No
tummy	Yes	No	No	No

As nouns, tummy is a hyponym of belly; that is, tummy is a word with a more specific, narrower meaning than belly:

Other hyponyms of belly include bay window, corporation, pot, potbelly.

belly (noun)	tummy (noun)
the underpart of the body of certain vertebrates such as snakes or fish	an enlarged and muscular saclike organ of the alimentary canal; the principal organ of digestion
the region of the body of a vertebrate between the thorax and the pelvis	slang for a paunch
a protruding abdomen
the hollow inside of something
a part that bulges deeply

belly (verb)	tummy (verb)
swell out or bulge out