migrating from XCP-ng (xen) to Proxmox (LVM)

create the VM, be the closest to the XCP-ng VM on Proxmox
detach the disk

export the VM with Xencenter in ova/ovf format (GUI)

extract the ova : 

tar -xvf <ova_file.ova>
this result in a vhd + ovf file

convert the filedisk format:

qemu-img convert -f vpc <disk_file.vhd> -O qcow2 <disk_filename.qcow2>

import the disk to the vm :  

qm importdisk <vm_id> <disk_filename.qcow2> <storage> -format qcow2

attach the disk to the vm, then boot