—Cloud Computing, born in the e-business context, and GRID computing, originated in the e-science context, are two different but similar paradigms for managing large sets of distributed computing resources. In the last few years there have been many efforts that aim at integrating them. One of the main problems linked to the use of the virtualization techniques adopted in clouds in the GRID and High Performance Computing context derives from the overheads in the virtualization layer. This paper introduces PerfCloud, a cloud environment built on the top of a GRID system, which lets the user to instantiate Virtual Clusters (VCs) that become part of the starting GRID. The architecture proposed integrates a set of services able to predict the performance (response time) of user applications on the newly generated VC taking into account the actual amount of computing and communication resources allocated to the VC, as well as the presence of the virtualization layer. The obtained predicti...