KVM virtual machines are complete virtualized machines, so configuration for NFS is as for a standalone machine. General instructions for Ubuntu are available at https://help.ubuntu.com/community/SettingUpNFSHowTo
The actual process followed:
sudo apt-get update
sudo apt-get upgrade
sudo apt-get install nfs-common
sudo nano /etc/default/nfs-common
sudo mkdir /mnt/nfs
sudo mount -t nfs nfsserver:/share_path /mnt/nfs
OpenVZ VMs are not complete VMs so there are some differences in how NFS is configured.
Notes on OpenVZ NFS setup are at: http://wiki.openvz.org/NFS