The Skylake-X and Kaby lake-X processors will share the same new platform, they will require a new motherboard with LGA2066 socket and will support DDR4 memory. The launch will be accompanied with the new Intel X299 chipset.
If earlier rumors are correct, Kaby Lake-X will be the lower-end version, limited to quad-core models with a TDP of up to 112W. Skylake-X on the other hand will have TDP range of up to 140W, and models with six, eight and ten cores.
There are some other differences too, Kaby Lake-X is reportedly limited to dual-channel DDR4, while Skylake-X has a quad-channel controller. Furthermore, Kaby Lake-X has up to 16 PCIe 3.0 lanes, whereas Skylake-X has up to 44 lanes.