The Glock 17C is a compensated (or more accurately, a ported) gun. There are little slots machined into the top of the barrel and slide near the muzzle, and when it is fired, hot powder gasses from behind the bullet jet out of these slots, acting like mini rocket motors, helping to keep the barrel/slide from flipping upward when the gun is fired. The effect isn't large, but it is tangible, and it does help keep the slide down nearer the target for faster repeat shots. Here is a photo of my G17C; you can see the little slots out near the end of the barrel:
I believe the XDm is a improved version of the original XD. I'm not really "up" on the latest models from Springfield Armory, but one of our other users will surely be along in a little while to explain the differences and correct me if I've said anything wrong.