extremely vs highly

extremely and highly both are adverbs.

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

As adverbs, extremely and highly are synonyms defined as:

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

extremely (adverb)	highly (adverb)
to an extreme degree	in a high position or level or rank
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