Gentoo Xen

Kernel configuration for Xen domain0:

Processor type and features  --->
   [*] Linux guest support  --->
      [*]   Enable paravirtualization code
      [*]     Xen guest support
      [*]       Support for running as a PVH guest
      [*]     Paravirtualization layer for spinlocks
Power management and ACPI options  --->
   [*] ACPI (Advanced Configuration and Power Interface) Support  --->
Bus options (PCI etc.)  --->
   <*> Xen PCI Frontend
[*] Networking support --->
   Networking options  --->
         <*> 802.1d Ethernet Bridging
	 [*] Network packet filtering framework (Netfilter) --->
            [*] Advanced netfilter configuration
            [*]   Bridged IP/ARP packets filtering
Device Drivers  --->
   Character devices  --->
      [*] Xen Hypervisor Console support
      [*]   Xen Hypervisor Multiple Consoles support
Device Drivers  --->
   [*] Block devices  --->
      <*>   Xen virtual block device support
      <*>   Xen block-device backend driver
Device Drivers  --->
   Input device support  --->
      [*] Miscellaneous devices  --->
         <*>   Xen virtual keyboard and mouse support
Device Drivers  --->
   [*] Network device support  --->
      <*>   Xen network device frontend driver
      <*>   Xen backend network device
Device Drivers  --->
   Graphics support  --->
         Frame buffer Devices  --->
            <*> Xen virtual frame buffer support
Device Drivers  --->
   Xen driver support  --->
         [*] Xen memory balloon driver
         [*]   Scrub pages before returning them to system
         <*> Xen /dev/xen/evtchn device
         [*] Backend driver support
         <*> Xen filesystem
         [*]   Create compatibility mount point /proc/xen
         [*] Create xen entries under /sys/hypervisor
         <*> userspace grant access device driver
         <*> User-space grant reference allocator driver
         <*> Xen PCI-device backend driver
         <*> Xen ACPI processor
         [*] Xen platform mcelog
         [*] Xen symbols

You can install pygrub with the USE flag on the xen-tools package, which is a python wrapper to the old GRUB installs. Alternatively, you can use the newer pvgrub instead.

emerge xen-pvgrub xen-tools xen

Once you've emerged the packages, there will be a xen.gz kernel located in /boot. You'll need to update your bootloader to use that as the kernel. Your original kernel is passed in as a module.

A sample GRUB legacy configuration:

title Xen Gentoo Linux 4.1.12
root (hd0,0)
kernel /boot/xen.gz
module /boot/vmlinuz-4.1.12-gentoo root=/dev/sda3

Update the Xen services to start on boot:

rc-update add xencommons default
rc-update add xenconsoled default
rc-update add xenstored default

If you want Xen domains to start on boot, put the configuration files in /etc/xen/auto, and add xendomains to the startup sequence:

ln -s /etc/xen/configs/openbsd-5.9_amd64 /etc/xen/configs/auto/
rc-update add xendomains default

Update /etc/inittab:

c0:2345:respawn:/sbin/agetty 38400 hvc0 linux

Reload your new inittab:

telinit q