On the PC FoV is available separately, as a digital download. On the Xbox it is too large to be released as DLC, and Larian could not get it published as a disk version (only the German PC release got a disk version of FoV).
There were too many changes in DKS (redesigned flying fortresses, changes to enemy placement and loot, etc) to maintain save compatibility. Lar mentioned recently the possibility of creating a tool to import D2:ED saves and export them to the FoV section of DKS, but it would have to wait until the FoV to DKS conversion patch is done. If such a utility does get released, I don't know if an Xbox version would be feasible.
