If you have the gmfn's from the HVM domU (I'am not sure where you get this
from if you want to keep your guest "pure" HVM) and the domU id, you can map
the domU memory into the dom0 kernel memory with

struct vm_struct area;
area = alloc_vm_area(num_pages * PAGE_SIZE);
direct_kernel_remap_pfn_range(((unsigned long)) area->addr, gmfn,
                               PAGE_SIZE*num_pages, prots, domU_id);