On a machine running with Xen kernel:

1) run virt-manager
2) choose para-virtualization
3) install ax30 as guest OS. Choose Max Installation.
4) in a terminal, boot the guest OS (AXS3) with the following command:

xm create -c NAME_OF_GUEST_OS

5) in the guest OS, modify  the grub, setting xen kernel as default kernel in the guest OS.
6) You can now use virt-manager to launch your guest OS, you do not  need to use "xm creat NAME_OF_GUEST_OS -c" option anymore.