How can I make NFS connection secure? Remote server is on the internet, and not in local network. The server has no firewall and connected to internet directly.
The server has no firewall
If the remote system is not secure (and an absence of any firewalling makes that decidedly suspect) then its doesn't matter what you do to your connection - you can't be confident of security. This is particularly true for an NFS server where authentication is not tied into the underlying protocol.
If you're assertions are correct (which I find astonishing) then find out who configured the server this way and shoot them before they can do any more harm.
If you need access to NFS across the internet, use a VPN (IPSEC, SSL tunnel, SSH tunnel, even pptp) and BLOCK all direct internet access (other than the secure connection) on the server.