guinea pig vs cavy

guinea pig and cavy both are nouns.

Word	Noun	Adjective	Verb	Adverb
guinea pig	Yes	No	No	No
cavy	Yes	No	No	No

As nouns, cavy is a hypernym of guinea pig; that is, cavy is a word with a broader meaning than guinea pig:

guinea pig: stout-bodied nearly tailless domesticated cavy; often kept as a pet and widely used in research
cavy: short-tailed rough-haired South American rodent

guinea pig (noun)	cavy (noun)
stout-bodied nearly tailless domesticated cavy; often kept as a pet and widely used in research	short-tailed rough-haired South American rodent
a person who is subjected to experimental or other observational procedures; someone who is an object of investigation