Usage documentation is on qvm-service manual page. There are also described predefined services.

Under the hood enabled service in VM is signaled by file in /var/run/qubes-service. This can be used to implement almost enable/disable per-VM switch controlled by dom0. Adding support for systemd services is pretty simple:

1. Disable old service: systemctl disable <service name>
2. Create /etc/systemd/system/<service name>.service file containing:

.include /lib/systemd/system/<service name>.service
    [Unit]
    ConditionPathExists=/var/run/qubes-service/<service name>

1. Enable new service: systemctl enable <service name>.

This will cause service to be started only when you enable it with qvm-service for given VM.