Machine virtualization as a way to support server consolidation, run applications in secure and isolated environments and support legacy applications is becoming increasingly popular. However, such virtualization imposes a performance cost on the system. Some of this cost can be mitigated by better cooperation between the operating system and virtual machine monitor. As a result, a good kernel analysis tool is crucial to allowing operating system and virtual machine monitor developers to improve performance in this way.This paper studies a kernel monitoring framework that we have implemented within a virtual machine monitor, which can be used to monitor guest operating systems. Such an implementation has several advantages, such as strong isolation of the monitoring system from the kernel, the ability to be dynamically enabled and better visibility due to the virtualization of hardware.