Linux – iptables and virtual machines of KVM, external ip

Try these rules on your gateway:

-A PREROUTING -i $VPN_INTERFACE -p tcp -m tcp --dport 80 -j DNAT --to-destination $INTERNAL_IP_DESTINATION:80
-A POSTROUTING -o $VPN_INTERFACE -j SNAT --to-source $VPN_IP_OF_SERVER
-A POSTROUTING -o $LAN_INTERFACE -j SNAT --to-source $LAN_IP_OF_SERVER

1st. one makes the port forwarding.
2nd. makes the source nat so every packet comming out of the VPN uses the server IP
3rd. makes the packets forwarded to the internal IP, have the soure nat of the server lan ip

There are three potential problems I see (contary to the other answer I don't see anything that would cause a "loop" even in the unedited version of your question).

1. IP forwarding must be enabled.
2. After being natted and placed back on the network the packet may fall victim to source address filtering as it looks very much like a spoofed packet.
3. Responses to packets that go through a NAT must go through the same NAT so the reverse translation can be performed. Otherwise the client will get a response with the wrong source IP/port which it is likely to drop (if it has not already been dropped by reverse path filtering).

You can work arround points 2 and 3 by using a SNAT or MASQURADE rule in addition to the DNAT but if you do that then you lose the original source IP of the traffic. That will make abuse control very difficult.

Another soloution to points 2 and 3 would be to set up a VPN between the two servers. Then use DNAT to forward traffic over the VPN and source IP based routing to bring the replies back to the NAT.