We run a mixed bag of Citrix XenServer and Gentoo/Xen virtualization platforms in our company. In general, we use XenServer where we have shared storage (SAN, iSCSI or whatever) and Gentoo/Xen where we want to use local storage.
With XenServer, we can get detailed utilization statistics for both the Dom0 and the various DomUs on the platform. We also monitor the DomUs via SNMP (Cacti). We can get stats from the Gentoo/Xen DomUs using SNMP or SAR or whatever else, but we aren't able to get good usage stats from the Dom0. Without this, we're not able to assess how effectively we're using the hardware and when we need to look at building out.
I'm sure this must be a solved problem, but my Google-Fu has let me down somewhat, so I'm hoping someone here might have some ideas.