Described systems and methods enable a computer security module to protect a set of guest virtual machines against computer security threats. In some embodiments, the computer security module receives introspection notifications from the protected VM, each such notification indicating that a particular trigger event (e.g., a system call) has occurred during execution of guest software within the respective VM. In some embodiments, delivering a notification comprises suspending execution of guest software and switching the processor to executing a notification handler forming part of the computer security module. In some embodiments, the computer security module may indicate to the processor a selected subset of events which trigger introspection notifications.