Can you have cloud computing without virtualization? I don't think so. Some have stated to me that they believe a cloud could b contrived without virtualization but I don't see it. Cloud computing is all about being able to expand or contract an enterprise on demand and as a service. Without deployable virtual images there is no mechanism for doing this efficiently.
I suppose that you could argue that clouds do not necessarily include the OS or the hardware and that you could scale by use of templates and configuration files to clone systems. That is cloning. Cloning, spawning, parallel processing and other mechanisms for creating capacity and processing power dont have the scope that a full cloud implementation has. Cloud computing is an administration paradigm that may share or even employ some or all of these other techniques but can include more.
I guess the biggest thing that sets cloud computing apart from cloining or spawning is that cloud computing is a paradigm for a flexible distributed computing platform. Cloning and spawing are techniques as is Virtualization.Clouds are entire managed infrastructures where virtulized systems are simply tools and cloning is a function of products.