The verb is "to know." To know by heart" is a separable phrasal verb meaning to know something very well. "Heart" is a noun. The prepositional phrase "by heart" completes the sense of the phrasal verb (as an adverbial).

"to know something by heart" is to have it memorized.

There is no verb in Standard English for "heart". In informal or slang, it means "to love". E.g. I heart New York. I suggest you master standard English first before delving wholeheartedly into slang.