The workload balancing engine (WLB) was introdcued within XenServer 5.6 FP1 introduced support for simple chargeback and reporting. The chargeback report includes, amongst other data, the following: the name of the VM, and uptime as well as usage for storage, CPU, memory and network reads/writes. You can use the Chargeback Utilization Analysis report to determine what percentage of a resource (such as a physical server) a specific department within your organization used.
see http://bit.ly/2sK6siq