When I try to export data from quickbooks, its exporting in XLSX format. Is it possible to export in XLS or CSV format ?

Use an exporter tool

https://get.transactionpro.com/qbo

Another option, purchase a 3rd party conversion service.

Good day, suvadeep. 

I'd be glad to help you export data in CSV format in QuickBooks Online (QBO).

The option to export a CSV file from QBO isn't available. What we can do is move the transaction out of QBO to an Excel file. From there, you can change the file format to a CSV file. 

I've added a screenshot for your reference. 

I've added this article to learn more about how to get your data out of QuickBooks: Export data out of QuickBooks Online.

Keep your post coming if you need more help exporting data out of QuickBooks. I'm always here to help. Have a great day!