Passo 1: can convert your VDI disk image to a RAW disk VBoxManage clonehd centos6.vdi centos6.img -format RAW
Passo 2: Refer to the centos6.vbox file, and create the configuration file for Xen VM. You can do this manually or create a shell script that would extract the necessary fields and parse them into Xen configuration file.
cat /etc/xen/centos6
name = "centos6"
uuid = "f220932f-8750-4f5b-b220-8050210e9835"
memory = 1024
vcpus = 1
bootloader = "/usr/bin/pygrub"
on_poweroff = "destroy"
on_reboot = "restart"
on_crash = "restart"
disk = [ "file:/var/lib/xen/images/centos6.img,xvda,w" ]
vif = [ "mac=00:16:3e:46:e6:fb,bridge=xenbr0" ]
extra="console=hvc0"