For a host that executes one or more guest virtual machines (GVMs), some embodiments provide a novel virtualization architecture for utilizing a firewall service virtual machine (SVM) on the host to check the packets sent by and/or received for the GVMs. In some embodiments, the GVMs connect to a software forwarding element (e.g., a software switch) that executes on the host to connect to each other and to other devices operating outside of the host. Instead of connecting the firewall SVM to the host's software forwarding element that connects its GVMs, the virtualization architecture of some embodiments provides an SVM interface (SVMI) through which the firewall SVM can be accessed to check the packets sent by and/or received for the GVMs.