Description of problem:

When installing Fedora Server 25 PV guest under Fedora Server 25 dom0 using virt-install or virt-manager the install works OK.

But the image cannot be booted up. The issue is that the kernel that is installed is the non-PAE and Xen PV drivers are compiled only in PAE (and 64-bit) kernels.

Which means we get this useless message:
virsh # start Fedora-32
error: Failed to start domain Fedora-32
error: internal error: libxenlight failed to create new domain 'Fedora-32'
Version-Release number of selected component (if applicable):


How reproducible:
100%

Steps to Reproduce:
1. Install Fedora 25. Install Xen, libvirt and virt-manager (dnf install xen libvirt virt-manager), reboot machine.
2. virt-manager, create new guest, select Xen PV guests
3. Point to the URL of the exploded tree: https://download.fedoraproject.org/pub/fedora/linux/releases/24/Server/i386/os/ and start installing
4. Install finishes, guest is rebooted and we get the: "
error: internal error: libxenlight failed to create new domain 'Fedora-32'"

Actual results:

Error in libxenlight.

Expected results:

No errors. Normal guest startup (like 64-bit PV guests).

Additional info:

I figured out it was the kernel issue by looking at the libvirt logs:

# tail /var/log/libvirt/libxl/libxl-driver.log 
2016-11-15 10:24:32 EST libxl: warning: libxl_bootloader.c:416:bootloader_disk_attached_cb: bootloader='/usr/bin/pygrub' is deprecated; use bootloader='pygrub' instead
2016-11-15 10:24:37 EST xc: error: panic: xc_dom_core.c:701: xc_dom_find_loader: no loader found: Invalid kernel
2016-11-15 10:24:37 EST libxl: error: libxl_dom.c:629:libxl__build_dom: xc_dom_parse_image failed: No such file or directory
2016-11-15 10:24:37 EST libxl: error: libxl_create.c:1223:domcreate_rebuild_done: cannot (re-)build domain: -3

Not much help there. Lets mount the image
[root@dhcp-whq-twvpn-2-vpnpool-10-159-175-2 ~]# losetup /dev/loop1 /var/lib/libvirt/images/Fedora.img 
[root@dhcp-whq-twvpn-2-vpnpool-10-159-175-2 ~]# losetup 
NAME       SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE                                       DIO
/dev/loop1         0      0         0  0 /var/lib/libvirt/images/Fedora.img                0
/dev/loop0         0      0         1  0 /srv/tftpboot/Fedora-Server-dvd-i386-24-1.2.iso   0

and get the kernel:
[root@dhcp-whq-twvpn-2-vpnpool-10-159-175-2 ~]# pygrub -i /dev/loop1
Using <class 'grub.GrubConf.Grub2ConfigFile'> to parse /grub2/grub.cfg
WARNING:root:grub2's saved_entry/next_entry not supported
WARNING:root:Unknown directive else
WARNING:root:grub2's saved_entry/next_entry not supported
WARNING:root:Unknown directive menuentry_id_option
WARNING:root:Unknown directive else
WARNING:root:Unknown directive menuentry_id_option
... snip..
linux (kernel /var/run/xen/pygrub/boot_kernel.CfF3K9)(ramdisk /var/run/xen/pygrub/boot_ramdisk.W_fXFl)(args "root=/dev/mapper/fedora-root ro rd.lvm.lv=fedora/root rd.lvm.lv=fedora/swap rhgb quiet LANG=en_US.UTF-8")
# file /var/run/xen/pygrub/boot_kernel.CfF3K9
/var/run/xen/pygrub/boot_kernel.CfF3K9: Linux kernel x86 boot executable bzImage, version 4.5.5-300.fc24.i686 (mockbuild.fedoraproject.org, RO-rootFS, swap_dev 0x5, Normal VGA

Well, nonPAE kernel! That won't boot as Xen PV guest (the PV drivers are not built in with
non-PAE kernels).