Hello.
I install Ubuntu 14.4 in VirtualBox and then use below command to install Xen :

sudo apt-get install xen-hypervisor-amd64

Then

sudo sed -i 's/GRUB_DEFAULT=.*\+/GRUB_DEFAULT="Xen 4.4-amd64"/' /etc/default/grub

Then

sudo sed -i 's/TOOLSTACK=.*\+/TOOLSTACK="xm"/' /etc/default/xen
sudo mv /etc/grub.d/10_linux /etc/grub.d/50_linux

then

sudo nano /etc/default/grub

GRUB_CMDLINE_XEN="dom0_mem=512M"
# Save and close

Then

update-grub
reboot


Ubuntu by default booted into Xen and show something to me and then "(initramfs)" but how can I boot into Ubuntu?


Thank you.

Problem Solved with Adding More Virtual CPUs and Memory but another problem exist.
My system is very Slow and in VirtualBox HDD light is always on

It may be that your host can support more integrated support for your client as in direct hardware support. When you then install a virtual machine within a virtual machine, I'm not sure that support follows. You may be using emulated processing.

The best test would be to dual boot to a linux and then run xen.

If you wish you may put the maximum resources to both clients available.

Consider edit's to original posts rather than answering your own post. It keeps your post on top that way.