OpenNebula is a fully open-source cloud management platform, with excellent performance and scalability to manage tens of thousands of virtual machines, and with the most advanced functionality for building virtualized enterprise data centers and private cloud infrastructures. OpenNebula is the result of many years of research and development in efficient and scalable management of virtual machines on large-scale distributed infrastructures. Its innovative features have been developed to address the requirements of business use cases from leading companies in the context of flagship international projects in cloud computing. OpenNebula is being used by many supercomputing and leading research centers to build HPC and science clouds for hosting virtualized computational environments, such as batch farms and computing clusters, or for providing users with new "HPC as a service" resource provisioning models. The talk describes how to design a cloud architecture with OpenNebula and its innovative features to enable the execution of flexible and elastic cluster and high performance computing services on demand while reducing the associated cost of building the datacenter infrastructure.