Hemma is a location adverb, hem is a direction adverb OR a noun ("a home")
Thus, it's "jag går hem" but "jag är hemma". But in the sentence above, it's a noun - "this is my home".
Does that help?
I'm not sure what an adverb "of är" means, but I suppose so. Hemma means you're at the place of home, while hem means you're heading in the direction of home.