HW - runs smoothly at 222 mhz while SW sometimes staggers even at 333,plus HW has superb image quality.
KM - Kernel Mode - for use with the 1.5 kernel. If you have a fat PSP with kernel 1.5 available you can run both KM and UM versions.
UM - User Mode - for kernels 2++. If you have a fat psp on the newest 3.71 m33 firmware and you didn't install the 1.50 addon, then use this. Use this as well if you have the slim PSP.
cya
Raziel-chan