A Wizard of Earthsea is fantasy: it takes place in another world where magic exists and dragons are real. It's a coming-of-age story: Ged grows up from a child to a man. And it's a quest story for most of the book: Ged's major goal from Chapter 4 to the end is to stop his shadow from hurting anyone.
You might agree with us on that. But there's one additional possible genre: parable. The story can be boiled down to a lesson: Ged grows up and as he does so, he realizes that this negative thing that he doesn't want is actually a part of him – it's his shadow. Just as Ged has to acknowledge this shadowy part of him so it doesn't control him, we might read here a parable for all of us: when we deny a negative part of ourselves, that negative part might become dangerous.
(Warning! Incoming public service announcement: In case you are planning on taking this parable to heart, please note that A Wizard of Earthsea doesn't say you need to give in to your dark side – only that you have to recognize it as part of you. In fact, recognizing it as part of you might help you keep it in check. That seems to be the way it works for Ged, at least.)