ESX / XenServer 6 on ESXi
edit /etc/vmware/config file on the ESX host
add line: vhv.enable = "TRUE" (for ESXi 5.0 it's vhv.allow = "TRUE")
create new custom VM with Hardware Version 8 & OS RedHat Enterprise Linux 6.0 x64
once VM has been created, edit settings
Options tab > Guest Operating System > select Other and VMware ESXi 5.x (experimental)
mount ESX / XenServer iso image
right-click VM and just under Edit Settings ... click Upgrade Virtual Hardware
check VM Hardware Version is now vmx-09
make sure promiscuous mode is enabled on the vSwitch to allow VMs to communicate with the rest of the network

Related Links: 
- Running Nested VMs - http://communities.vmware.com/docs/DOC-8970
- How to Enable Support for Nested 64bit & Hyper-V VMs in vSphere 5
   http://www.virtuallyghetto.com/2011/07/how-to-enable-support-for-nested-64bit.html
- How to Enable Nested ESXi & Other Hypervisors in vSphere 5.1
   http://www.virtuallyghetto.com/2012/08/how-to-enable-nested-esxi-other.html

Hyper-V on ESX 5.1 - 
- Create a "Microsoft Windows Server 2012 (64-bit)" VM, using Typical Configuration
- right-click the new VM and Upgrade Virtual Hardware to Version 9 (vmx-09)
- tweak the virtual machine's VMX, adding these 4 lines:

mce.enable = TRUE
hypervisor.cpuid.v0 = FALSE
featMask.vm.hv.capable = “Min:1″
vhv.enable = TRUE

June '13 update - try changing just 2 lines in .vmx file
     guestOS = "winhyperv"
     featMask.vm.hv.capable = "Min:1"
Source: VMware ESXi 5.1 can run Microsoft Hyper-V Server 2012 VMs

April '14 update
    hypervisor.cpuid.v0 = "FALSE"
    mce.enable = "TRUE"
Source: Fix "Hyper-V cannot be installed: A hypervisor is already running

be sure it's assigned to a network where the vSwitch has Promiscuous Mode set to Accept

File and printer sharing on Hyper-V host is disabled by default - to enable it run:
netsh advfirewall firewall set rule group="File and Printer Sharing" new enable=Yes

Disable firewall
netsh advfirewall set currentprofile state off