If source and destination host are both proxmox VE you can just use vzdump - see the documentation -
https://pve.proxmox.com/wiki/Backup_and_Restore.
If the source system is not a Proxmox VE you can export the disk from the source system and use the importdisk command of qm (see `man qm`)
If the target system is not a Proxmox VE you can use the move_disk feature (also available in the GUI), to create a qcow2 or raw image on a directory storage, and use the other system's import facilities.