What is different between the CNAME and DNS-01 challange?
100%
There are no similarities between them.
so what is difference here for using CNAME method i need to add txt for every renewal
I think you need to be a lot clearer in your questions in order to get more helpful advice. I'll try to cover an overview of possible validation methods, though:
Let's Encrypt follows CNAME records when validating records, just like any system querying DNS does. The current challenge types that Let's Encrypt supports include DNS-01, which yes requires a new TXT record for each renewal (which generally would be done by your client automatically updating your DNS server when making a request). Some people use NS or CNAME records to point to a system like acme-dns, a special-purpose DNS server just for answering such challenges.
Let's Encrypt (along with various standards bodies and other CAs) are working on implementing a new challenge type called DNS-PERSIST-01, which allows for leaving a single TXT record in place across multiple renewals.
Some other CAs might instruct you to make CNAME to their systems, either as the DNS record that they're directly checking for, or just like any other service provider so that they can respond to challenges on your behalf. Again, you might need to give a lot more details about what "CNAME method" you're asking about if you want more specific information.
The is no CNAME authentication method.
You can augment authentication methods by using a CNAME to delegate your FQDN to some other FQDN; but no CA will [currently] validates merely because you created a CNAME record in DNS.
Well, not through ACME (directly, though yes of course CNAME records get followed).
But a CA can validate a domain name through method 3.2.2.4.7 DNS Change, which allows for a CNAME with a random value.
This is not quite true. The Baseline Requirements allow the Random Token to be placed in a CNAME record, e.g. as part of a subdomain label. However, few CAs actually do this, and no ACME CAs do it. But the mere existence of this possibility does make OP's question even more ambiguous.
I had what seems to be a similar question a couple of weeks ago, maybe this thread will help you understand CNAME better.