By default, KVM gives your virtual machine a realtek rtl8139 Ethernet adapter, with an ancient 100Mbit/Second speed, we all need gigabit Ethernet adapter for the KVM guest.
The answer is changing the string rtl8139 with virtio in the XML file of the virtual machine, then installing the drivers
The steps i use are
Run virtual machine with the realtek adapter to download the other adapter’s driver
once the adapter is there, shutdown the virtual machine guest (Windows guest), then edit the xml of the guest, and restart libvirtd
start the KVM guest again
open with VNC, start the device manager, install the driver you downloaded.
You are good, the adapter should report the speed of 10Gbit/second (10 gigabit per second)
One annoying thing is that all windows drivers come in a big ISO file, you probably just want the driver you need.
I will add the download links in the coming few days, but you can get them right now if you like from fedora, the fedora windows guest drivers should work on any linux distribution (Debian, ubuntu, etc…)