@dthenot THANK YOU !!

@olivierlambert : I was able to do a passthrough and assign to VM. But then I needed to detach it. I did
/opt/xensource/libexec/xen-cmdline --delete-dom0 xen-pciback.hide

then followed by

/opt/xensource/libexec/xen-cmdline --set-dom0 "xen-pciback.hide=(0000:03:00.0)(0000:03.00.1)(0000:03:00.2)"

I had subsequently assigned it to a VM. Then I

Then reboot. The first time I did it it worked fine.

However, now its not working anymore.

My boot config looks like

menuentry 'XCP-ng' {
search --label --set root root-fwztre
multiboot2 /boot/xen.gz dom0_mem=7584M,max:7584M watchdog ucode=scan dom0_max_vcpus=1-16 crashkernel=256M,below=4G console=vga vga=mode-0x0311
module2 /boot/vmlinuz-4.19-xen root=LABEL=root-fwztre ro nolvm hpet=disable console=hvc0 console=tty0 quiet vga=785 splash plymouth.ignore-serial-consoles xen-pciback.hide=(0000:03:00.0)(0000:03.00.1)(0000:03:00.2)
module2 /boot/initrd-4.19-xen.img

[16:07 xilinx-xcp ~]# xl pci-assignable-list
libxl: error: libxl_pci.c:427:libxl_device_pci_assignable_list: Looks like pciback driver not loaded

Deleting and recreating the pciback.hide is not solving the problem.