Converter does not support xva format. One way would be to import in xen server, power the VM on and converted it as physical (caveat: if it is Linux, it should be with hardware virtualized kernel).
Another way would be to export it in ovf instead of xva and import it directly to vSphere (or WS) w/o converter.
AFAIK xen convert can import to xen server but not to vmware.
HTH
Plamen