So my doubt is can we completly disable ipv4 address and use only ipv6 address.
Yes. Without a doubt, this is possible. IPv4 is more widely supported, particularly in regards to automatic address assignment (most commonly implemented as "DHCP" in IPv4). Many earlier devices do not support automatically assigning the IPv6 address (or may not support the same approach that is used by another IPv6 device), so you might need to manually set the network settings. However, once done, IPv6 ought to work pretty well, just as IPv4 ought to.
And next [...] is how a only ipv6 host can communicate with only ipv4 enabled host.
...and...
so how an ipv6 host can communicate with ipv4 host
It can't do this, directly. The only way is for traffic to be relayed. A device in the middle, which might be called a proxy or a tunnel broker or a router, will need to receive traffic on one of the Internet Protocols, and translate the payload into packets that use the other Internet Protocol.
I do wish to clarify one aspect of this answer. Two devices with IPv6 traffic could communicate, using IPv6, by placing the IPv6 packets into IPv4 packets (which may be described as a tunnel) and then communicating with IPv4. However, I would say that such computers are not "only IPv6" if they are using IPv4 packets, so that network design doesn't really match the question you were asking. I'm bringing this up because this (stuffing packets of one type into a packet, or multiple packets, of another type) is what happens with some of the protocols you mention.
What type of tunneling is using for that
There are multiple approaches, some of which you've mentioned (6to4, 6in4). The primary reason that there are multiple approaches is that there are multiple computer programmers, who had some different preferences. There may be some minor differences, like one protocol having more overhead but being better at working with networks that use NAT to modify packets.
The most important detail is that both sides will need to use the same approach in order to be able to understand packets used by the other side. Some solutions might be easier on some operating systems, or supported by a specific device that could be a router.
ipv6.it is down, but trex.fi is still up. – Brian Minton – 2016-10-07T13:04:05.570