FYI - aka "avkdm" followuing this thread is Rod Smith, developer for reEFIne.
Actually, I'm Rod Smith, rEFInd's maintainer, not avkdm.
I don't know anything about ESXi or XenServer, so I'm afraid I can't answer the question about whether rEFInd can launch them. If they start via standard EFI boot loaders, then rEFInd should be able to do it. Likewise if they start via conventional BIOS boot loaders. If they launch after an OS kernel has launched, then it's up to the OS configuration to launch them.