I don't know about xVal but as far as the ASP.NET MVC side of things go, if you have a look at the source you'll see there are some static readonly strings that represent these values.
public static readonly string ValidationInputCssClassName = "input-validation-error";
public static readonly string ValidationMessageCssClassName = "field-validation-error";
public static readonly string ValidationSummaryCssClassName = "validation-summary-errors";
So either you edit the source to your liking (which may also fix xVal) or alter the outputting code to use different values somehow.
If xVal is using jQuery Validation (which I think it does...?) you could change it's default value for the 'errorClass' option. My answer here could help you along that path.
HTHs,
Charles
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