caviar vs hard roe

caviar and hard roe both are nouns.

Word	Noun	Adjective	Verb	Adverb
caviar	Yes	No	No	No
hard roe	Yes	No	No	No

As nouns, hard roe is a hypernym of caviar; that is, hard roe is a word with a broader meaning than caviar:

caviar: salted roe of sturgeon or other large fish; usually served as an hors d'oeuvre
hard roe: fish eggs or egg-filled ovary; having a grainy texture

Other hypernyms of caviar include roe.

caviar (noun)	hard roe (noun)
salted roe of sturgeon or other large fish; usually served as an hors d'oeuvre	fish eggs or egg-filled ovary; having a grainy texture