highly vs extremely

highly and extremely both are adverbs.

Word	Noun	Adjective	Verb	Adverb
highly	No	No	No	Yes
extremely	No	No	No	Yes

As adverbs, highly and extremely are synonyms defined as:

highly and extremely: to a high degree or extent; favorably or with much respect

highly (adverb)	extremely (adverb)
in a high position or level or rank	to an extreme degree
to a high degree or extent; favorably or with much respect	to a high degree or extent; favorably or with much respect
at a high rate or wage