Export the VM file
The VM must be prepared for export and compression. On the running VM, fill out the disk with 0s.
dd if=/dev/zero of=/mytempfile rm -f /mytempfile
And stop the VM afterwards
Then, Compress the VM. Note somewhere the uncompressed VM size, this will be required for the import. On the proxmox server, run the following command in the folder where the image is located. You need to replace the 127 with the id of the VM, and the 1 with the id of the disk
qemu-img convert -p -O qcow2 -c vm-127-disk-1.qcow2 vm-127-disk-1-compressed.qcow2
When this is finished transfer the compressed file where needed.
Import the VM file (LVM storage)
On the targeted proxmox, create a new VM with at least equivalent storage space. In the next steps we will write the previoulsy exported disk in this new image. Do not start it.
Then, uncompress the VM on a storage with enough space. If you do not know the uncompressed size, multiply the compressed size by 6 at least.
qemu-img convert -p -O raw vm-127-disk-1-compressed.qcow2 vm-127-disk-1.raw
Finally, copy the disk. Note that the VM and disk ids might be different for the new VM
dd if=vm-127-disk-1.raw of=/dev/pve/vm-999-disk-5.raw