Apple has been evaluating both OpenStack and KVM. Currently, it is only clear that they are going to use KVM, which has been positioned as a lower-cost alternative for VMware ESXi hypervisor and it is being promoted by companies such as IBM and Red Hat.
The move from VMware to KVM could help Apple save million of dollars in licensing costs as they scale up their cloud infrastructure, in the same way as other big technology companies have done, eg Amazon Web Services, Google and Facebook.
Apple’s use of Open Source isn’t something new. They have been talking about their use of Apache Mesos cluster-manager in Siri’s infrastructure.