Thanks for your input, Konstantin.

Some more context.

Vm was migrated from vSphere (using an external provider via network).
The VM has an application that communicates with an Oracle System on a second server (VM) running Linux.
With the Windows VM on vSphere, with vmxnet3 driver, the network performance of the application was about 5Gbs.

The Linux server is a VM on oVirt infra.
The reason to move the Windows VM to oVirt is to investigate if it can get better performance.
But after the migration test it seems that the application network performance is about 2Gbs, so far worse than on vSphere.

After these application results above, some bare tests with iperf3 were done.
On oVirt Linux -> Linux with VMs on two different hypervisors network performance is more than 9Gbs
Windows -> Linux on same hypervisor 2Gbs
Windows -> Linux on different hypervisor 1.5Gbs
Linux -> Windows almost 10Gbs

As suggested from the links you provided I tried iperf2, using EPEL iperf rpm for Linux VM and sourceforge iperf-2.1.8-win.exe for Windows.

With VMs on different hosts and transferring from Windows to Linux I got 9.3Gbs

So the problem is not the driver itself or VM configuration but probably the "legacy" application doesn't support multiqueue or any network performance optimizations that are available in the driver.

Gianluca