By default OpenVZ CT in proxmox don't allow run nfs-kernel-server. To enable it, you must to active the nfsd feature in the container.
This method is possible that don't work with NFSv4.
# vzctl stop CTID
# vzctl set CTID --feature nfsd:on --save
# vzctl start CTID
NOTE: CTID is means Container ID.
You can check if nfs-kernel-server is running:
# service nfs-kernel-server status
To show the export directories:
# showmount -e