I am testing XenServer 6.5 on a single host, and as a test I have exported and imported a few VMs in OVF format from my existing ESXi server. All good so far, but I was wondering: I have a hard drive full of ESXi VMs, is there any way to attach it to XenServer without having to export and reimport all VMs individually?
If it's VMFS, then you're out of luck.
VMFS is proprietary to VMware and the only way I know to access it other than from an ESX(i) host is with the VDDK, but even that might not be of any use to you.
You could, in theory map an NFS volume to the ESXi node, move the VMs there and then mount the same volume on the XenServer host as well. Not sure how nicely the two hypervisors will deal with it, but if you're very careful about which one has which VM in it's inventory, you might be OK.
If you have a large number of VMs, you could leverage PowerCLI to export all of them to OVF.