The virtual network (virbr0) used for Network address translation (NAT) which allows guests to access to network services. However, NAT slows down things and only recommended for desktop installations. To disable Network address translation (NAT) forwarding type the following commands:
KVM (Kernel-based Virtual Machine) is a full virtualization solution for Linux on x86 hardware containing virtualization extensions Intel VT or AMD-V. How do I install KVM under CentOS or Red Hat Enterprise Linux version 5.5?
virt-manager requires GUI locally and it is slow. On other hand virt-install is a command line tool for provisioning new virtual machines using the “libvirt” hypervisor management library. The tool supports both text based & graphical installations, using serial console, SDL graphics or a VNC client/server pair. The guest can be configured to use one or more virtual disks, network interfaces, audio devices, and physical host devices (USB, PCI). You can use the virt-install to create virtualized guests as follows to install FreeBSD or CentOS.