Use [UIWebView stringByEvaluatingJavascriptFromString:] method and get the values by executing the javascript.
Example: If your html is:
<html>
<body>
<input id="myId" type="text" value="TextValue"/>
</body>
</html>
Following code can get you the value of the text field:
NSString* value = [webView stringByEvaluatingJavaScriptFromString:@"document.getElementById('myId').value"];