some words about oSSI and Xen?

hi to all

i know that Xen is a solution for building virtual machine, but
i don't understand what is the aim in using Xen with HPC cluster such as 
openSSI?

if somebody could explain to me the aims and what benefits we could gain 
  ? i'm afraid that virtual machines slow down computing power?
but i may be wrong

thanks for answers to this basic question
ML

-- 
      Maurice Libes
Tel : +33 (04) 91 82 93 25            Centre d'Oceanologie de Marseille
Fax : +33 (04) 91 82 65 48            UMS2196 CNRS- Campus de Luminy, 
Case 901
mailto:[email protected]  F-13288 Marseille cedex 9
Annuaire : http://annuaire.univmed.fr/showuser.php?uid=libes

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