Hoya is marketed in the USA and Kenko in europe.
Not as far as I'm aware. Both brands are available generally. I think it is a positioning difference, with Hoya being the more premium and filter centric brand and Kenko providing a more value option as well as other accessories like extension tubes, teleconverters etc...
Canon 1D, 300D IR, 450D full spectrum, 600D, 5D2, 7D, 7D2, EF 35/2, 85/1.8, 135/2+SF, 70-300L, 100-400L
EF-S 10-18, 15-85, MP-E 65, Zeiss 50/2 macro, Sigma 50A, 150 macro, 120-300/2.8 OS, Samyang 8mm fisheye