Hi Arun! In terms of pattern-wise difference between CAT and XAT, the sections of Decision Making and GK are there in XAT, but not in CAT. Coming to the Decision-Making section, there is no specific preparation strategy. The questions are based on ethics and principles and no book or study material can teach you that. In this section, your conscience would be the biggest guide. Having said that, for acing this section, practice is the best way. Refer to the previous years' question papers and solve the questions and match with the answer keys. 

As far as GK is concerned, there is no alternative but to read newspapers and keeping yourself abreast with the current affairs on the internet, tv news etc. If you have not built the habit of newspaper yet, it is high time you do so as there is no alternative to it. Read one general daily and one business daily. Along with the reports, read the Editorial articles as well. You may find it boring or difficult to pay attention initially, but it's all about practice and habit.