protected string RenderPartialViewToString(string viewName, object model)
{
    if (string.IsNullOrEmpty(viewName))
        viewName = ControllerContext.RouteData.GetRequiredString("action");
    ViewData.Model = model;
    using (StringWriter sw = new StringWriter()) {
        ViewEngineResult viewResult = ViewEngines.Engines.FindPartialView(ControllerContext, viewName);
        ViewContext viewContext = new ViewContext(ControllerContext, viewResult.View, ViewData, TempData, sw);
        viewResult.View.Render(viewContext, sw);
        return sw.GetStringBuilder().ToString();
    }
}
Also refer: http://craftycodeblog.com/2010/05/15/asp-net-mvc-render-partial-view-to-string/