Many people have been in similar situations before, and, of course, it's not PVE specific.
You should find what is taking up the space. The usual culprit in PVE installation - "/var/lib/vz" which is the location of your "local" storage pool.
It...
In a default installation, your disk is divided into two parts at a high level: root filesystem and LVM pool for VM storage.
They both live on the same physical disk. They are both backed by LVM volume group.
Again, as part of the default...