In order to migrate, treat PROXOMOX VM as physical machine and do P2V conversion. i.e. the same way we’d convert a physical host.
Obtain a P2V iso image, connect it to the guest to be converted in PROXOMOX, and boot the guest under PROXOMOX using the P2V image. The P2V image runs the P2V client.
The P2V client reads local disks and sends data across the network to a conversion host.
The conversion host converts the image to run on KVM, and writes it out to a target RHEV or RHEL/libvirt hypervisor. Search our knowledgbase for P2V migration procedure.