vmdk to xva conversion is typically searched by users that are looking for a way to migrate virtual machines between VMware and Citrix Xen, which is of course doable, just not with a direct file conversion.
You have to convert whole virtual machine package, not just some of it's parts, but perhaps import of disk images to existing Citrix Xen machine is doable. In any case, XenConvert /Citrix Hypervisor conversion tool is what you are looking for. Unfortunately Citrix no longer provides download of XenConvert, so you have to look for alternative download locations to this discontinued software.
Other than that VMWare migration tools are also viable alternative.
Updated: October 10, 2023