sweet lemon vs Citrus limon

sweet lemon and Citrus limon both are nouns.

Word	Noun	Adjective	Verb	Adverb
sweet lemon	Yes	No	No	No
Citrus limon	Yes	No	No	No

As nouns, Citrus limon is a hypernym of sweet lemon; that is, Citrus limon is a word with a broader meaning than sweet lemon:

sweet lemon: lemon tree having fruit with a somewhat insipid sweetish pulp
Citrus limon: a small evergreen tree that originated in Asia but is widely cultivated for its fruit

Other hypernyms of sweet lemon include lemon, lemon tree.

sweet lemon (noun)	Citrus limon (noun)
lemon tree having fruit with a somewhat insipid sweetish pulp	a small evergreen tree that originated in Asia but is widely cultivated for its fruit