Try a search in these forums for "xdm". This question has been asked many times in the last few months. Here is my canned response to the last two threads dealing with this:
I own an XD-40 subcompact and an XDM-40 bi-tone.
Here are most, if not all, of the differences between an XDM an an original XD: 3 interchangeable backstraps, higher magazine capacity (without widening the grip, they actually shaved material off from inside the magazine well), slightly different ergonomics (tapered slide, wider sharper notches where your fingers grip the slide to rack it, lots more checkering or stipling on the grip), match grade barrel, slightly crisper trigger with a shorter reset (it feels awesome), slight serrations on the rear sight to help reduce glare, no trigger pull necessary to disassemble, and a slightly higher price tag. But I really like mine, it's extremely accurate, and as with all XD models, points naturally and feels great in the hand. The grip angle and width, dimensions, etc. honestly feel almost as good as a government 1911. I'm still under the 1,000 round count on mine, but I like it more and more each time I shoot it.