Although your Q has been answered a few times here in the Networking forum, I’ve answered and described Virtualization Networking many times in the Virtualization forum.
Briefly,
Although some have configured direct physical connections, for best flexibility practically all virtualization technologies (KVM, Xen, VMware, VBox, Docker, etc) create and use Linux Bridge Devices as virtual objects representing defined virtual networks. A LBD describes connecting through a specific network interface, and possible other features like the type of network connection (eg bridging, NAT, Host Only), network IP range, DHCP services. When you create LBDs for each type of virtual network, then all you need to do to switch from one network to another is to change which LBD to use in the Guest properties. Note that switching between networks is done <only> through the Guest properties, the network connection <within> the Guest remains unchanged, is always “the” default wired connection. This wired connection can be configured statically or if a DHCP service is available as a DHCP client. You can also connect multiple Guests to any LBD.
A LBD can be created a number of different ways. YAST will create LBD using the standard naming convention (br0, br1, etc) but is limited in what you can create easily.
You can create, manage and view (the last 2 very important) using the command line brctl utility. Although I rarely if ever create a bridge device using brctl, it’s a very important troubleshooting tool to view configured bridge devices on your machine.
You can create LBD using a virtual networking manager. If you were using VMware, you might use something like VMware Workstation. If you were using VBox, you’d use the standard VBox Manager. Since you are running KVM, I assume you also have libvirt (vm manager, vm install virt install) installed. You should create, configure and manage your virtual networks using vm manager. When vm manager creates a LBD, the naming convention is virbr0, virbr1, etc.
The above GUI tools are the standard ways to create and manage your virtual networking. Of course, there are command line alternatives if desired for all the virt technologies.
No matter how you create a LBD, once it has been created by whatever means, it’s available to be used by any and all virtual Guests on that machine no matter the virtualization technology.
If you are having specific problems setting up a LBD using vm manager, post your specific attempt (or state objectives), plus your attempt.
HTH,
TSU