NIST says that virtualization isn’t needed for cloud computing
What exactly is cloud computing? Until recently, there was some debate of that, but NIST's definition that appears in their SP 800-145, "The NIST Definition of Cloud Computing" now seems to be the definitive one, even if some vendors don't agree with it. Here's their list of the essential characteristics of cloud computing:
On-demand self-service. A consumer can unilaterally provision computing capabilities, such as server time and network storage, as needed automatically without requiring human interaction with each service’s provider.
Broad network access. Capabilities are available over the network and accessed through standard mechanisms that promote use by heterogeneous thin or thick client platforms (e.g., mobile phones, laptops, and PDAs).
Resource pooling. The provider’s computing resources are pooled to serve multiple consumers using a multi-tenant model, with different physical and virtual resources dynamically assigned and reassigned according to consumer demand. There is a sense of location independence in that the customer generally has no control or knowledge over the exact location of the provided resources but may be able to specify location at a higher level of abstraction (e.g., country, state, or datacenter). Examples of resources include storage, processing, memory, network bandwidth, and virtual machines.
Rapid elasticity. Capabilities can be rapidly and elastically provisioned, in some cases automatically, to quickly scale out, and rapidly released to quickly scale in. To the consumer, the capabilities available for provisioning often appear to be unlimited and can be purchased in any quantity at any time.
Measured Service. Cloud systems automatically control and optimize resource use by leveraging a metering capability at some level of abstraction appropriate to the type of service (e.g., storage, processing, bandwidth, and active user accounts). Resource usage can be monitored, controlled, and reported, providing transparency for both the provider and consumer of the utilized service.
Note that virtualization is not part of this definition. And although implementations of cloud computing often use virtualization, it's certainly possible to have implementations of cloud computing that don't use it. You often see vendors of virtualization technologies saying that their technology is an essential part of cloud computing, but while their technologies may be very useful in some particular implementations of cloud computing, they're certainly not an essential feature of it.