Looks like Wireguard dont work with virt-io network adapter with Windows as a guest on KVM

Took me many hours to get Wireguard working in a Windows server 2022 VM. After many hours i understood that large UDP packets disappeared when they should go from the guest to the host. UDP packets over 1000 bytes just disappeared. First i set MTU to 1000 in the Windows guest. That made the network work. When i switch the virtual network adapter to e1000e instead of virt-io the UDP packets stopped disappearing.

You can see the network connections names with

netsh int ipv4 show sub

You can change the MTU in windows with

netsh int ipv4 set sub <Connection name> mtu=<size> store=persistent

example

netsh int ipv4 set sub "Wi-Fi 2" mtu=1492 store=persistent