In this paper, the authors survey various technologies in the field of virtualization that are used for ‘On demand’ dynamic resource allocations. Cloud computing is one of the emerging areas in the various fields related to computer science and can be exercised for on demand dynamic resource allocation for providing reliable and guaranteed services to the consumer. Cloud systems are ‘on demand’ as they offer a way of providing required resources to consumers as and when they need it. It is pay-as-you-use manner service. In cloud systems, a pool of resources is shared by several users. So making these resources available in most optimal way is a challenging task. This paper reviews various techniques for how such dynamic resource allocations can be achieved in an optimal manner.