Can any one clarify this on xen cloud platform/xen server

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http://serverfault.com – As per XCP documentation, Xen Cloud Platform host is a 64 bit x86 server-class machine devoted to hosting multiple VMs. on blog.citrix.com they are mentioning about dom0 as It should be noted that dom0 is a 32-bit process i am little bit confused over these facts what is host and dom0 mentioning here.I thought dom0 is a 32 bit centos kernel and does host means xen hypervisor? if host means hypervisor which is based on 64 bit kernel then why it is not able to address more than 4GB for itself. Also How is the 32 bit kernel of dom0 depends on assigning more ram to xen server/XCP(on citr (HowTos)