difference between redirect and redirect inside modelandview

The statements:

return "redirect:/abc.htm"
return new ModelAndView("redirect:/abc.htm")

do the same thing: redirects the request to abc.htm. If a view name is returned that has the
prefix redirect:, this is recognized as a special indication that a redirect is needed. The rest of the view name will be treated as the redirect URL.

With the statement

return "redirect:/abc.htm"

you can only return the redirect view name.

With ModelAndView you can return both model and view in a single return value:

ModelAndView modelAndView =  new ModelAndView("redirect:/abc.htm");
modelAndView.addObject("modelAttribute" , new ModelAttribute());
return modelAndView;

But the attribute value will not be available in the new redirect request that the client(browser) will make for the URL /abc.htm. The best use of ModelAndView is when you forward the request to a new URL, so that you can return both model and view together in a single return value. For redirect scenarios, if you want to pass attributes, you should use RedirectAttributes.