Virtual Machines (VM) allow the execution of various operating systems and provide several functionalities which are nowadays strongly appreciated by developers and administrators (isolation between applications, flexibility of resource management, and so on). As a direct consequence, ``virtualization'' has become a buzz word and a lot of ``virtualization'' solutions have been proposed, each providing particular functionalities. Goldberg proposed to classify virtualization techniques in two models (Type-I and Type-II), which does not enable the classification of latest ``virtualizations'' technologies such abstraction, emulation, partitioning and so on. In this document, we propose an extension of the Goldberg model in order to take into account and formaly define latest ``virtualization'' mechanisms. After giving general definitions, we show how our proposal enables to rigorously formalize the following terms: virtualization, emulation, abstraction, partitioning, and identity. We also demonstrate that a single virtualization solution is generally composed by several layers of virtualization capabilities, depending on the granularity of the analysis.