As well as allocating a portion of physical memory for its own private
use, Xen also reserves s small fixed portion of every virtual address
space. This is located in the top 64MB on 32-bit systems, the top
168MB on PAE systems, and a larger portion in the middle of the
address space on 64-bit systems. Unreserved physical memory is
available for allocation to domains at a page granularity. Xen tracks
the ownership and use of each page, which allows it to enforce secure
partitioning between domains.
Each domain has a maximum and current physical memory allocation. A
guest OS may run a `balloon driver' to dynamically adjust its current
memory allocation up to its limit.