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kvm (with a little patch) supports it, running it right now with 5 guests and have 53K pages which are shared.
# cat/sys/kernel/mm/ksm/pages_sharing
53714
That's ~200MB for about 1,5GB memory used on the host.
Now I can't figured out how many times those pages are shared, so I can't calculated the actual memory saved (it's between 200MB and 4x200MB).