FoV is only available separately on the PC. It is too large for DLC on the Xbox, and Larian could not get just the add-on published (even for the PC version, the only physical edition of FoV was the German release). On the Xbox the only way to get FoV is as part of DKS.

On the PC, D2:ED saves are compatible with FoV, and playing the game there is a seamless transition into the add-on. With DKS there were too many changes to maintain saved game compatibility, but you can start a new level 35 character at the beginning of the add-on content (in DKS or with D2:ED + FoV).
In another topic recently Lar mention the possibility of a save conversion utility to transfer D2:ED saves into the FoV section of DKS, but that would at least have to wait until they get the FoV to DKS patch out. If they do release such a tool, I don't know how feasible an Xbox version would be.

There have been a couple FoV vs DKS topics, but unfortunately the forum search ignores search terms less than 4 characters. I'm pretty sure there is at least 1 newer topic, that may contain more up to date feedback from people who have played more...

How different is the remastered edition?

D2:ED + FoV vs DKS?