Dear Maintainer,
like already mentioned in Bug#1098798, the Linux kernels in Trixie that are installed by linux-image-amd64 are not capable of booting on a Xen hypervisor. This is an issue with the config used to compile the kernel. I'm not certain which config causes that issue, but compiling the kernel from upstream based on a sufficiently old 6.12.x config produces a kernel without any such issues. The error message that's produced is
Loading Linux 6.12.28-amd64 ...
error: not xen image.
Loading initial ramdisk ...
error: you need to load the kernel first.
Press any key to continue...
VM type is "pv".
Best regards
Richard