This command may return varied results depending on the hypervisor you are using.

The dommemstat [domain] [--period (sec)][[--config][--live]|[--current]] displays the memory statistics for a running domain. Using the --period option requires a time period in seconds. Setting this option to a value larger than 0 will allow the balloon driver to return additional statistics which will be displayed by subsequent domemstat commands. Setting the --period option to 0, will stop the balloon driver collection but does not clear the statistics in the balloon driver. You cannot use the --live, --config, or --current options without also setting the --period option in order to also set the collection period for the balloon driver. If the --live option is specified, only the running guest's collection period is affected. If the --config option is used, it will affect the next boot of a persistent guest. If --current option is used, it will affect the current guest state

Both the --live and --config options may be used but --current is exclusive. If no option is specified, the behavior will be different depending on the guest's state.

#virsh domemstat rhel6 --current