That song is a great power rock ballad by the way. To answer your question, the meaning is the same between the two phrases. Dario is right in that "I still love you" is commonly used. You could also say "I'm still in love with you", or "I love you still". "I'm still loving you" is actually grammatically incorrect but was written this way I am sure because it fits the music or rhyme scheme of the song better. This is one of the problems in learning a language through song lyrics.

There is really no difference in meaning between the sentences other than "I still love you" is much more often used in dialogue. I don't think I've ever said to someone "i'm still loving you." It sounds a bit awkward.