Same as for qcow2 ... if nobody is working on it upstream, it'll not going to be in PVE. ZFS was very easy to integrate, because it is already present in the filesystem, no other filesystem or storage technology available in PVE (besides btrfs) does this. LVM uses a non-VM friendly snapshot method, so it cannot be used to transfer incremental changes - neither does qcow2.
However, there are technologies in development in QEMU that will work with any underlying storage (e.g. COLO), but it currently neither finished nor stable and therefore not integrated in PVE.