A virtual machine requires a network interface to connect to a network. The nic_interfaces element defines the supported NIC types available. Each nic_interface depends on the drivers available for different types of virtual machines. VirtIO drivers are available for Red Hat Enterprise Linux 4.8 and above, and for Windows virtual machines. Windows supports rtl8139 without the need for drivers. Other Linux machines, or earlier versions of Red Hat Enterprise Linux, use e1000 or rtl8139.

<capabilities>
    <version major="3" minor="3">
        ...
        <nic_interfaces>
            <nic_interface>e1000</nic_interface>
            <nic_interface>virtio</nic_interface>
            <nic_interface>rtl8139</nic_interface>
            <nic_interface>rtl8139_virtio</nic_interface>
        </nic_interfaces>
        ...
    </version>
</capabilities>