Server virtualization creates new security threats while turning the hypervisor into a network black hole, hiding traffic from traditional hardware defenses--problems a new breed of virtualization-aware security software tackles head-on.
That leaves us with a limited number of major products for hypervisor network security. Two of them, VMware's own vShield and Juniper's vGW (Virtual Gateway, acquired from Altor), use the APIs provided under VMware's VMsafe security program. Cisco, the other big player in this market, bases its technology around the proprietary Nexus 1000V virtual switch, which was developed in cooperation with VMware but isn't dependent on VMsafe. Cisco hasn't completely hitched itself to VMware's wagon; it has hinted that the technology will be usable with other hypervisors.
If you run a non-VMware hypervisor, you should be looking at Vyatta's Network OS product, which works with Citrix XenServer and Red Hat KVM, and, like VMware's vShield Edge, includes NAT and DHCP servers. Vyatta also adds a sophisticated routing engine with support for IPv4 and IPv6 dynamic routing protocols like BGP, OSPF, and RIP.