How to improve post views count display?

Yes you can. You have to check if the post view count is higher than 1000, is so, then round it and return it:

function getPostViews($postID) {
    $count_key = 'post_views_count';
    $count = get_post_meta($postID, $count_key, true);
    if($count=='') {
        delete_post_meta($postID, $count_key);
        add_post_meta($postID, $count_key, '0');
        return "0 View";
    }
    if ($count > 1000) {
        return round ( $count / 1000 ,1 ).'K Views';
    } else {
        return $count.' Views';
    }
}