1. Umount the filesystem and check it with e2fsck
umount /var/lib/vz
e2fsck -f /dev/mapper/pve-data
2. Re-size the file system of /dev/mapper/pve-data (Let said to 500GB)
resize2fs /dev/mapper/pve-data 500G
3. Resize the Logical Volume of LVM
lvresize -L 500G /dev/pve/data
4. Check the size of the Logical Volume
lvdisplay /dev/pve/data
5. Mount the filesystem and check the size
mount /var/lib/vz
df -h
Enjoy it.
Reference:
http://www.tcpdump.com/kb/os/linux/lvm-resizing-guide/shrink.html