Using session wrapper for accessing session data
Do not use Hungarian notation for constants, keep it in PascalCase. If you like to differentiate the names from the properties you can name in form of {PropertyName}Key, like UserIdKey.
Do prefer .net aliases over the full type name, like Int32 -> int, String -> string.
Avoid repetition of word, like User.UserId and SessionHelper.GetSession.... Unless you are worried about mixed up with the foreign key in your DTO, but it is not the case here.
UserModel (if it is User, that is fine)(you weren't really consistent with the code) should be renamed to CurrentUser :

// which one makes more sense ?
UserModel.Name = userName;
CurrentUser.Name = userName;

HttpContext.Current.Session shows up quite frequently in SessionHelper, you could use a property to shorten it :

private static HttpSessionState CurrentSession
{
    get { return HttpContext.Current.Session; }
}





The SessionHelper class : 

public class SessionHelper
{
    private static HttpSessionState Session
    {
        get { return HttpContext.Current.Session; }
    }

    public void SaveValue(string key, string value)
    {
        Session.Add(key, value);
    }
    public static string GetStringValue(string key)
    {
        if (Session == null || Session[key] == null)
            return null;

        return HttpContext.Current.Session[key].ToString();
    }
    public static int GetIntValue(string key)
    {
        if (Session == null || Session[key] == null)
            return 0;

        int result;
        return int.TryParse(Session[key].ToString(), out result) ? result : 0;
    }
}


If you can get your hand on C# 6, consider this version :

public class SessionHelper_c6
{
    private static HttpSessionState Session =&gt; HttpContext.Current.Session;

    public void SaveValue(string key, string value)
    {
        Session.Add(key, value);
    }
    public static string GetStringValue(string key)
    {
        return Session?[key]?.ToString() ?? null;
    }
    public static int GetIntValue(string key)
    {
        var value = Session?[key]?.ToString() ?? null;
        int result;
        return value != null &amp;&amp; int.TryParse(value, out result) ? result : 0;
    }
}