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:
/etc/systemd/system/<service name>.service.d/30_qubes.conf file
containing (you may need to create a directory for this file first):
[Unit] ConditionPathExists=/var/run/qubes-service/<service name>
This will cause service to be started only when you enable it with qvm-service for given VM.