The need for the real-time interactive co-visualization of 3D urban environments on a Web-based virtual Globe arises naturally in GIS but it still remains challenging due to the complexity of city models and their huge data sizes which largely overload the computational power and memory capacity of client devices. Especially on the Web, the visualization of city models makes their rendering not real-time because of the lack of content adaptation and progressive data transmission. This paper presents technical solutions for the co-visualization of massive city models in a Web-based virtual globe, allowing navigation over 3D cities on the globe in real-time. The volume of 3D city data, such as building data, does not allow us to render them directly, nor to keep them in the main memory. We propose to use not only a hierarchical presentation of geo-spatial data to create a chunk-based multiple resolution data structure which reduces complexity of the geometry being rendered; but also a view dependent algorithm so that only small subsets of 3D city models are streamed progressively in real-time and kept in client memory to contribute efficiently to the rendered image. Experimental results show that we can navigate over 3D cities on the Globe in real-time.