hi,
i like to clarify.
ip a
ens33: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc
ovs-br0: inet 192.168.42.99/24
virbr0:inet 192.168.122.1/24
vnet0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc

virsh net-list
Name State Autostart Persistent
----------------------------------------------------------
default active yes yes
ovs-network active yes yes

default mean virbr0?

virsh dumpxml testvm
<interface type='bridge'>
<mac address='52:54:00:b7:bb:6f'/>
<source network='ovs-network' bridge='ovs-br0'/>
<virtualport type='openvswitch'>
<parameters interfaceid='bb12f32a-7b64-4206-988a-dbfe5f4519ad'/>
</virtualport>
<target dev='vnet0'/>
<model type='virtio'/>
<alias name='net0'/>
<address type='pci' domain='0x0000' bus='0x00' slot='0x03' function='0x0'/>

When systemctl start libvirtd the bridge virbr0 is created.
In the above dump the VM testvm is accessed by ovs-br0 and not by virbr0.right?
I can set up that all VMs to use ovs-br0 or part of them to use virbr0.right?
what is the advantage of using ovs-br0?
tnx

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Yes.
All future VMs. It might be possible to move VMs to ovs-br0, but I don't know whether it can be done, what effort is required, and whether it makes sense.
virbr0 is the default libvirt bridge. Whoever added ovs-br0 wanted a virtual network separate from libvirt's default network. Perhaps a network whose properties can be modified without modifying the default network.