In this paper, the authors present their experiences of building Linux/RTOS (Real-Time Operating System) hybrid operating environments on Xen and Gandalf Virtual Machine Monitors (VMMs). Xen is a popular open source VMM while Gandalf is their in-house virtual machine monitor that was designed and implemented from scratch to be a simple yet extremely lightweight VMM. They ported an RTOS to both Xen and Gandalf, which were enabled to host multiple RTOSes along with Linux. One significant advantage of employing a VMM to construct such a hybrid environment is that OSes executed on a VMM can be spatially and temporally protected from each other.