1. What happens to after Naturalisation, I can travel with my Indian passport?
Technically you are no longer an indian citizen and you shouldn't travel on the Indian passport (though probably you can).
https://www.hcilondon.in/renunciation.html
The Indian Citizenship Act, 1955, does not allow dual citizenship. Holding Indian passport/acquiring Indian passport/travelling on Indian passport after acquisition of foreign citizenship constitutes an offence under the Indian Passport Act, 1967, and attracts penalties. The Government of India has prescribed imposition of penalty on a graded scale for the violation of Passport Rules, depending on number of trips made on Indian passport after acquiring foreign nationality (with a three month grace period from the date of acquiring foreign nationality i.e. the date of Naturalisation certificate), renewal of Indian passport after acquiring foreign nationality and retention of Indian Passport for more than 3 years after acquiring of foreign nationality. The date on the Naturalisation Certificate will be treated as date of acquiring foreign nationality and hence, it is mandatory for the applicants to produce his / her Naturalisation Certificate for obtaining Surrender Certificate.
2. Visa for XYZ country wil be still valid or I need to apply again after British passport?
I don;t think that visa will be valid as your circumstances (citizenship) have changed. Possibly the XYZ country may fall under a visa waiver country list in which case, you wouldn't need to apply for a visa once you get your British passport - check here
https://www.gov.uk/foreign-travel-advice