1. A virtualized computer system comprising: one or more processors; one or more memories; one or more virtual machines, each virtual machine providing an execution-support environment in which a guest operating system executes; one or more applications, each executing within an execution-support environment provided by one of the one or more guest operating systems that each executes within an execution-support environment provided by one or the one or more virtual machines; and a virtual-machine monitor, executing on one or more of the one or more processors, that provides an execution-support environment for, and manages execution of, the one or more virtual machines, the virtual-machine monitor including two or more different virtualization support subsystems, each of which, when currently selected for one or more virtual machines, is invoked to handle, differently from the other of the two or more different virtualization support subsystems, attempts by guest operating systems to execute privileged instructions, a virtualization-performance monitor, including instrumentation included within of a set of stored processor instructions that implement the virtual-machine monitor, that collects performance data and characterizes, using the collected performance data, the performance of each of the one or more different virtualization support subsystems with respect to each of the one or more virtual machines, and stores the collected performance data and characterizations of the performance of each of the one or more different virtualization support subsystems on one or more of the one or more memories; and a virtualization-support-subsystem selector that uses the performance data collected by the virtualization-performance monitor and the characterizations of the performance of each of the one or more different virtualization support subsystems to select a virtualization support subsystem for each virtual machine.