[Mashiro]

A proper filesystem like EXT4 on a proper virtualization technology such as KVM by default has a certain amount of its space (a few percent usually) reserved to prevent a total system breakdown when the actual partition runs out of space. This reserved space is deducated from usable space and therefore usually decreases the amount of total usable disk space.

See here for more information and ways to reduce the amount of reserved space: https://wiki.archlinux.org/index.php/ext...ved_blocks

1 GB less sounds like 1% reserved space has been set. Maybe it has been done automatically during the upgrade to KVM. The OpenVZ ploop filesystem basically emulates a real filesystem (compared to the older simfs) and also allows to reserve space. So maybe the amount of space that was reserved before has been left as is during the upgrade.

df -h should also help a little more. Usually the system creates some partitions that are automatically removed on shutdown and created again when booting. Some of them are created in RAM (tmpfs).