You can configure XenCenter to generate alerts when CPU, network I/O or disk I/O usage go over a specified threshold on a managed server or virtual machine. By default, the alert repeat interval is 60 minutes, but you can configure this as well.
Performance alerts will appear in the System Alerts window (accessed by clicking on System Alerts at the top right corner of the XenCenter window). In addition, you can have performance alerts emailed to you along with other serious system alerts: see XenCenter system alerts.
To configure performance alerts: