Hello, there is this backup script, Contribution: OpenNebula backup script for QCOW2 datastores, but actually without LVM support, but can be extended to support LVM.
It uses external snapshots with block commit, so can be used also for VMs using block devices.
https://wiki.libvirt.org/page/Live-disk-backup-with-active-blockcommit
after creating snapshot you can just dd if=/path/to/lvm/image of=/path/to/backup/file
after it you blockcommit external snapshot back to the original vm image.