There has recently been an explosion of work in the human-robot interaction (HRI) community on the use of mixed, augmented, and virtual reality. We present a novel conceptual framework to characterize and cluster work in this new area and identify gaps for future research. We begin by introducing the Plane of Interaction: a framework for characterizing interactive technologies in a 2D space informed by the Model-View-Controller design pattern. We then describe how Interactive Design Elements that contribute to the interactivity of a technology can be characterized within this space and present a taxonomy of mixed-reality interactive design elements. We then discuss how these elements may be rendered onto both reality- and virtuality-based environments using a variety of hardware devices and introduce the Reality-Virtuality Interaction Cube: a three-dimensional continuum representing the design space of interactive technologies formed by combining the Plane of Interaction with the Reality-Virtuality Continuum. Finally, we demonstrate the feasibility and utility of this framework by clustering and analyzing the set of papers presented at the 2018 VAM-HRI workshop.