To upgrade your Oracle VM Server, you must upgrade the Oracle VM Manager first to the dedicated version and then upgrade Oracle VM Server also using Oracle VM Manager using the yum repository or by using the CD.
In this case: upgrade your Oracle VM Manager from the version 3.2.1 to the latest version and then upgrade Oracle VM Server from the version to the latest version.
Note: Oracle VM Manager and Server should have the same version.
I hope this can help you
Thanks WadhahDAOUEHI, very helpful