Hi Teddy,
thanks for the analysis and your brief explanation.
IOMMU should be properly activated:

(XEN) [ 0.221843] AMD-Vi: IOMMU Extended Features:
(XEN) [ 0.222606] - Peripheral Page Service Request
(XEN) [ 0.223366] - NX bit
(XEN) [ 0.224123] - Guest APIC Physical Processor Interrupt
(XEN) [ 0.224889] - Invalidate All Command
(XEN) [ 0.225649] - Guest APIC
(XEN) [ 0.226412] - Performance Counters
(XEN) [ 0.227178] - Host Address Translation Size: 0x2
(XEN) [ 0.227940] - Guest Address Translation Size: 0
(XEN) [ 0.228681] - Guest CR3 Root Table Level: 0x1
(XEN) [ 0.229416] - Maximum PASID: 0xf
(XEN) [ 0.230140] - SMI Filter Register: 0x1
(XEN) [ 0.230867] - SMI Filter Register Count: 0x1
(XEN) [ 0.231596] - Guest Virtual APIC Modes: 0x1
(XEN) [ 0.232316] - Dual PPR Log: 0x2
(XEN) [ 0.233024] - Dual Event Log: 0x2
(XEN) [ 0.233727] - Secure ATS
(XEN) [ 0.234424] - User / Supervisor Page Protection
(XEN) [ 0.235126] - Device Table Segmentation: 0x3
(XEN) [ 0.235826] - PPR Log Overflow Early Warning
(XEN) [ 0.236514] - PPR Automatic Response
(XEN) [ 0.237198] - Memory Access Routing and Control: 0x1
(XEN) [ 0.237881] - Block StopMark Message
(XEN) [ 0.238558] - Performance Optimization
(XEN) [ 0.239234] - MSI Capability MMIO Access
(XEN) [ 0.239906] - Guest I/O Protection
(XEN) [ 0.240570] - Enhanced PPR Handling
(XEN) [ 0.241231] - Invalidate IOTLB Type
(XEN) [ 0.241886] - VM Table Size: 0x2
(XEN) [ 0.242537] - Guest Access Bit Update Disable
(XEN) [ 0.252988] AMD-Vi: IOMMU 0 Enabled.
(XEN) [ 0.253820] I/O virtualisation enabled

I have also read about the problem with UEFI several times.
Now I am going to try to find a system with "legacy" boot mode.