@olivierlambert Now, that I have PVHVM it seems I want to have the latest PVH(v2) instead. 
https://wiki.xen.org/wiki/Xen_Project_Software_Overview#Summary
However, "PVH (v2) requires guests with Linux 4.11 or newer kernel." - and Debian 9.x runs on kernel 4.9.
https://lists.xenproject.org/archives/html/xen-devel/2018-01/msg00540.html
Too bad..
Converting a Debian VM from PV (paravirtualized) to PVHVM (PV-on-HVM drivers)
Well, as often there's a little bit more to do than just running a oneliner. 
So I created a summary based on information found here, elsewhere and the one I added to successfully convert a Debian VM 9.x from PV to PVHVM on XCP-ng 7.4.x.
Reconfigure GRUB defaults in /etc/default/grub (Debian). Make sure GRUB_TERMINAL is uncommented and set to console (disabling graphical modes, which caused a black screen in XenCenter / VNC for me):
#GRUB_CMDLINE_LINUX="console=hvc0"
GRUB_TERMINAL=console
Update GRUB:
update-grub
Poweroff the vm:
poweroff
Retrieve the UUID of the virtual machine:
xe vm-list name-label=your_vm_name_goes_here params=uuid
Set HVM boot mode:
xe vm-param-set uuid=your_vm_uuid_goes_here HVM-boot-policy="BIOS\ order"
Set local disk (c) and cdrom (d) as boot options:
xe vm-param-set uuid=your_vm_uuid_goes_here HVM-boot-params="cd"
Clear pygrub as boot loader:
xe vm-param-set uuid=your_vm_uuid_goes_here PV-bootloader=""
Clear the display arguments:
xe vm-param-set uuid=your_vm_uuid_goes_here PV-args=""
Find the UUID of the interface of the virtual disk:
xe vm-disk-list uuid=your_vm_uuid_goes_here
Set the disk device (VBD) as bootable:
xe vbd-param-set uuid=your_vbd_uuid_goes_here bootable=true
Start the VM and verify your VM actually is running in PVHVM mode:
Hope this saves someone else a little bit of time. 
Great, thank you, Olivier. I'll give it a try and will report back to you. Would be nice to finally also have Debian VMs running in PVHVM. CentOS 7.5 works flawlessly. It's about time then..
Any suggestion on how to convert to PVHVM? I'll give it a try with clone/template VM. And then run some benchmarks. I am curious, too - whether it has changed. Last test I ran was with XS 7.0 as far as I remember. Most guests are PVHVM, but Debian I kept PV.
Web, Mail, no HPC. That's what I thought. It's a quite modern system: Dell R730XD with Intel Xeon E5 2600 v3 and Intel C610 series chipset.
PV mode. It seems to be still faster than HVM.
Dear kind members,
a quick word of warning after an unpleasant experience:
Debian Stable with linux-image-4.9.0-6-amd64 - 4.9.88-1+deb9u1 works flawlessly under XCP-ng 7.4.1
however, Debian Stable linux-image-4.9.0-7-amd64 - 4.9.110-1 enables (K)PTI (Kernel page-table isolation) which causes this Kernel to crash: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=903767
mitigation: either wait for 4.9.110-2 , to be released for Debian Stable soon or disable (K)PTI with: extra = 'elevator=noop pti=off'
Cheers
Alexander
Thank you for all your kind and fast replies. I sincerely apologize for replying so late. Once I've built the drivers I'll make them available here.
Dear XCP-ng team,
First of thank you for your hard work and patience. After upgrading from XenServer 7.2 to XCP-ng 7.4.1 everything works flawlessly, except the Quad (2x 1Gbit, 2x up to 10Gbit) NIC on a Dell R730XD:
exact model: QLogic 57800S Quad Port 2, 1GB x 2, 10Gb rNDC Base-T
issue:
Firmware on NICs is the latest & greatest: 14.05.01
However, the driver in XenServer & XCP-ng is older: 7.14.6.0 (the version since XenServer 7.3.0)
Apparantly this issue is known to Citrix:
Instead of blaming the hardware which came with the popular server - and which worked flawlessly until now, means with XenServer 7.2 that is - I'd like to solve the issue which I assume to be outdated drivers.
Dell just published the latest Qlogic network drivers for Linux - sources and binaries - in June 2018:
Sources:
https://downloads.dell.com/FOLDER04919194M/1/Source.tar
Binaries (RHEL6/RHEL7/SLES):
https://downloads.dell.com/FOLDER04919193M/1/Drivers.tar
As both XenServer and XCP-ng don't use the Kernel version from RHEL6 or RHEL7, but 4.4.0 in case of XCP-ng 4.7.1, those binaries won't help.
Afaik one has to use the XenServer DDK to build fitting binaries, correct?
Current DDK version on Citrix.com for XenServer 7.5.0 is still 7.3.0. No newer and no XCP-ng-specific DDK yet, correct?
What are the steps required to make sure not only I build the drivers for my personal use, but the updated driver sources are part of future XCP-ng builds? I'd like to help.
Thank you very much.
Kind regards,
Alexander