Uh... Judging by the name "Sylveon", I would assume without a shadow of a doubt that it's flying, since it very clearly comes from the word "Sylph", which is a small, invisible air pixie thing.
Well, you could follow that train of thought a bit more, since "nin" in japanese can mean "person", and the bows, ribbons, and the fur boa on its head make it more human.