What does "ERROR Out of memory" mean on a Red Hat Enterprise Linux 5 Dom0?

Release Found : Red Hat Enterprise Linux 5 with a Xen kernel

Issue

When creating a new virtual machine with a x86_64 vCPU, the creation fails with the error

ERROR Out of memory: xc_dom_boot_mem_init: can't allocate low memory for domain

Note: error can be found in log file: /var/log/xen/xend-debug.log