Mainly there are 2 types: those with inner metal brace and those made out of just the flexible plastic guards. The result is, that in the event of a crash one of those (the metal protected) will prevent your hands from being hit and the other won't.

The main manufacturers are: Ufo Plastics and Acerbis, in fact the original Yamaha ones are made by Acerbis (with the metal inserts).

Ktm original also fit, handy if your xtx is orange.