This paper presents our Linux/RTOS hybrid operating environment constructed upon Gandalf VMM. Gandalf can host multiple RTOSes along with Linux, and RTOSes and Linux execute within their own isolated protection domains; thus, they can be spatially and temporally protected from each other. We design Gandalf from scratch as a simple and efficient VMM in order to minimize overheads incurred by virtualization. The simplicity and efficiency are achieved by the hybrid of para- and nearly full-virtualization approaches. The implementation of the presented hybrid operating environment is on the PC/AT compatible platform with the Intel IA-32 processor with μITRON as an RTOS. From the measurement results, we make clear that the benefits of using a VMM to construct a hybrid environment exceed shortcomings by showing the impact on performance is limited.