See also GitHub - Xmegaduino/Xmegaduino: Xmegaduino fork of Arduino
There are quite a few hits on a web search for "xmega arduino."
Till relatively recently, the more interesting xMega chips have been plagued with availability issues and bugs 
Since the Arduino core, by it's nature, is unlikely to be able to take advantage of the fancy peripherals, about the only advantage of xMega is the 32MHz clock rate, which is pretty much "not compelling" for the effort required for a new core and new hardware (especially since ... ARM!) (ok, "big" xMega chips are now running lower cost than big MEGA chips...)