Xen 4.11 on MacPro (early2008) NOT booting, blank screen. Dom0 alone boots OK; Debian buster, LVM on Raid1, efi

[eugen@xxxxxx]

It's a fresh installation, using debian installer, then installing
the xen-system packages. See hardware and versions details later below.

Booting the Debian system, the dom0 alone, works great.
Booting with Xen Hypervisor stops at a blank screen, as described below:
All the Grub2 commands seems to be executed ok: the echo messages are
displayed, I can see them fine (like 'Loading initial ramdisk ...',
including a last echo debug message I put there myself. Immediately after
that, the screen goes blank and stays blank forever. My Samsung TV
(hdmi) doesn't complain about signal lost or something.

There are no log messages in /var/log or any indication that domO under
Xen was loading/starting. Also no network presence (like ping, ssh).

I've tried many command line options like vga=current,keep or vga=ask or
iommu=off or ... No one made any difference... everything was exactly as
before - for example I wasn't asked to choose the vga mode. (I edited
the commands at runtime, in the grub menu).

Any idea of what's going on? 

Any debug tips on how to find more about the problem? how to display
more debug messages? Maybe a serial console on USB? With console=dbgp?
not much info about that. I have a Raspberry PI that can do UART... I've
read https://wiki.xenproject.org/wiki/Xen_Serial_Console and it seems
that I don't have any physical serial port or a technology that provides
one.

I would appreciate any thought/guess on this...

Hardware:
MacPro (early 2008: Macpro3,1)
2x Intel Xeon 4562
ATI Radeon HD 2600 (2xDVI ports) with firmware from
firmware-amd-graphics package
connected to a Samsung TV (dvi-hdmi cable)

Software:
Debian buster (10.4) amd64 - using debian-installer
LVM on RAID1
Using EFI 64 bits
Grub 2.02
Linux kernel 4.19.0-9-amd64
Xen-system 4.11.3

Regards,
   Eugen