Hey Tushar.

The eligibility criteria for Xaviers Entrance test 2018 was that you must have passed or should be appearing in the qualifying exam in 2018 itself. If you are appearing in the qualifying exam later than June 30, 2018, then you will not be eligible to apply for St. Xavier’s BMS Entrance Test 2018. 

You can expect a similar eligiblity criteria for 2019 also, hence you won't be eligible.

Best of luck!