Use the esxtop tool on the ESXi host to monitor the performance of your virtual machines. 

The following articles provide useful information on esxtop:

• Performance Monitoring Utilities: resxtop and esxtop: http://pubs.vmware.com/vsphere-51/index.jsp?topic=%2Fcom.vmware.vsphere.monitoring.doc%2FGUID-A31249BF-B5DC-455B-AFC7-7D0BBD6E37B6.html

• Interpreting esxtop Statistics: http://communities.vmware.com/docs/DOC-9279