By default, the XLR8 behaves just like an Arduino, right down to its timing and 16 MHz clock. However, the XLR8's clock frequency can be changed, and various functions -- like the PWMs -- can be implemented using XBs ("acceleration blocks") inside the FPGA.
For example, the Arduino Uno doesn’t have a floating-point unit, which means it has to clunk along implementing floating-point operations using lots and lots of simple instructions. In turn, this means it executes floating-point operations verrryyy sslllooowwwlllyyy. By comparison, the XLR8 has dedicated floating-point hardware programmed directly into the FPGA's fabric. Check out this video comparing an Arduino Uno and an XLR8 using floating-point math to generate Mandelbrot fractal images.
The reason I'm waffling on about this here is that I just heard that the creators of the XLR8 will be hosting a free webinar on Friday, September 30, at 3:00pm Central Daylight Time.
Also of interest is the fact that these little scamps will be presenting a 2-hour tutorial demonstrating A Novel Hands-On Approach to Learning FPGA Design at the forthcoming Embedded Systems Conference (ESC) in Silicon Valley, December 6-8, 2016.
I'm planning on attending both of these events -- I'll be the outrageously handsome one in the Hawaiian shirt (LOL) -- hopefully I'll see you there.