The choice of vif-plugging for the NVP Plugin depends on what version of libvirt you are using (this assumes you are using NVP for security groups and VM spoof prevention).
When using libvirt (version < 0.9.11): libvirt_vif_driver=nova.virt.libvirt.vif.LibvirtOpenVswitchDriver
When using libvirt (version >= 0.9.11): libvirt_vif_driver=nova.virt.libvirt.vif.LibvirtOpenVswitchVirtualPortDriver
When using XenServer: xenapi_vif_driver=nova.virt.xenapi.vif.XenAPIOpenVswitchDriver