About OVA/OVF I think this format doesn't keep snapshots, right? Otherwise you would tell me!
Unfortunately, none of the options (backup/restore or export/import) will maintain snapshots.
OVA/OVF takes much space? It's compressed ? Or equivalent to hard disk size and memory size?
OVF exports - besides some metadata - the guest's used disk space. OVA is an archive which contains all of the OVF files in a compressed format (comparable to a .zip archive).
André