RHV currently uses IPv4 for internal communications and does not use/support IPv6. Using IPv6 at the virtual machine level is fully supported though, provided that youre using a guest operating system thats compatible.
With RHV 3.2 the manager UI now reports IPV6 addresses in addition to IPV4 addresses
RHV 3.5 introduces network custom properties, which may be used to assign IPV6 addresses to host interfaces using the vdsm-hook-ipv6 hook; see https://www.mail-archive.com/users@ovirt.org/msg22375.html.