Oracle Linux is free to download, use and distribute and is provided in a variety of installation and deployment methods. Installation media (ISO images) for Oracle Linux (and Oracle VM) are freely available from the Oracle Software Delivery Cloud. Ubuntu Server › Whether you want to configure a simple file server or build a fifty thousand-node cloud, you can rely on Ubuntu Server and its five years of guaranteed free upgrades.
From Red Hat:
A base Red Hat Enterprise Linux server provides fully-virtualized guest support for Red Hat Enterprise Linux 3, 4, and 5. Third party operating systems may also be hosted. Fully-virtualized guest support requires hardware features provided by Intel Virtualization Technology and AMD-V processors. Para-virtualized guests can be configured on any 32-bit x86 (with PAE support), 64-bit x86, or Itanium2 system. With Red Hat Enterprise Linux Advanced Platform the number of guest operating systems that may be hosted is unlimited, constrained only by physical hardware resources.
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From Red Hat:
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A base Red Hat Enterprise Linux server provides fully-virtualized guest support for Red Hat Enterprise Linux 3, 4, and 5. Third party operating systems may also be hosted. Fully-virtualized guest support requires hardware features provided by Intel Virtualization Technology and AMD-V processors. Para-virtualized guests can be configured on any 32-bit x86 (with PAE support), 64-bit x86, or Itanium2 system. With Red Hat Enterprise Linux Advanced Platform the number of guest operating systems that may be hosted is unlimited, constrained only by physical hardware resources.