<?xml version="1.0"?>
<ownershipDocument>
    <schemaVersion>X0101</schemaVersion>

    <documentType>4</documentType><periodOfReport>2003-06-06</periodOfReport>
    <issuer>
        <issuerCik>0001058057</issuerCik>
        <issuerName>MARVELL TECHNOLOGY GROUP LTD</issuerName>
        <issuerTradingSymbol>MRVL</issuerTradingSymbol></issuer><reportingOwner><reportingOwnerId>
            <rptOwnerCik>0001134607</rptOwnerCik>
            <rptOwnerName>BANATAO DIOSDADO P</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>  </rptOwnerStreet1>
            <rptOwnerStreet2>  </rptOwnerStreet2>
            <rptOwnerCity>  </rptOwnerCity>
            <rptOwnerState>  </rptOwnerState>
            <rptOwnerZipCode>  </rptOwnerZipCode>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>1</isDirector><isOfficer>0</isOfficer><isTenPercentOwner>0</isTenPercentOwner><isOther>0</isOther></reportingOwnerRelationship></reportingOwner><nonDerivativeSecurity><securityTitle><value>Common Stock</value></securityTitle><transactionDate><value>2003-06-06</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>M</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>25000</value></transactionShares><transactionValue><value>.0367</value></transactionValue><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>1400000</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value></value></natureOfOwnership></ownershipNature></nonDerivativeSecurity><nonDerivativeSecurity><securityTitle><value>Common Stock</value></securityTitle><transactionDate><value>2003-06-06</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>S</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>25000</value></transactionShares><transactionValue><value>34.4438</value></transactionValue><transactionAcquiredDisposedCode><value>D</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>1375000</value><footnoteId id="F1"/></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value></value></natureOfOwnership></ownershipNature></nonDerivativeSecurity><derivativeSecurity><securityTitle><value>Stock Option (Right to Buy)</value></securityTitle><conversionOrExercisePrice><value>.0367</value></conversionOrExercisePrice><transactionDate><value>2003-06-06</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>M</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>25000</value></transactionShares><transactionValue><value>0</value></transactionValue><transactionAcquiredDisposedCode><value>D</value></transactionAcquiredDisposedCode></transactionAmounts><exerciseDate><value>1988-08-08</value><footnoteId id="F2"/></exerciseDate><expirationDate><value>2006-07-22</value></expirationDate><underlyingSecurity><underlyingSecurityTitle><value>Common Stock</value></underlyingSecurityTitle><underlyingSecurityShares><value>609600</value></underlyingSecurityShares></underlyingSecurity><postTransactionAmounts><sharesOwnedFollowingTransaction><value>609600</value><footnoteId id="F3"/><footnoteId id="F4"/><footnoteId id="F5"/><footnoteId id="F6"/></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value></value></natureOfOwnership></ownershipNature></derivativeSecurity><footnotes><footnote id="F1">Reporting Person also has indirect beneficial ownership in 1,676,747 shares held by the Diosdado Banatao &amp;amp; Maria C. Banatao Trust; as well as 680,680 shares held by Tallwood Partners LLC.</footnote><footnote id="F2">These shares are fully vested and immediately exerciseable.</footnote><footnote id="F3">Summary of Derivative Securities: The Reporting Person owns directly four Stock Options (Right to Buy) with different vesting schedules described as follows: (i) 180,000 shares granted on January 28, 1997 at $0.05 per share which such shares are fully vested and immediately exerciseable; (ii) 30,000 shares granted on June 26, 2000 at $15.00 per share, vests 20% on 06/26/01 and 500 shares per month from 07/26/01 through 06/26/05; (iii) 6,000 shares granted on June 21, 2001 at $20.58 per share, vests 500 shares per month from 07/21/05 through 06/21/06; and (iv) 6,000 shares granted on June 21, 2002 at $21.59 per share, vests 500 shares per month from 07/21/06 through 06/21/07.  Stock Options are immediately exerciseable but not fully vested, and until fully vested, if exercised, would be subject to repurchase by the Company as to the unvested shares in the event of termination of the reporting person's services as a non-employee director of the Company. </footnote><footnote id="F4">Summary of Derivative Securities: The Reporting Person also has indirect ownership of a pre-paid forward contract (Right to Sell) held by Tallwood Partners LLC (the &amp;quot;Partnership&amp;quot;).  The contract provides that the Partnership deliver a certain number of shares at the end of the contract depending on the Issuer's common stock price on such date. The contract for 500,000 shares was entered into on 11/30/01 with a settlement date on 03/05/04.  Exact pricing terms are determined in accordance with the contract.</footnote><footnote id="F5">Summary of Derivative Securities: The Reporting Person also has indirect ownership of six pre-paid forward contracts (Right to Sell) held by the Diosdado &amp;amp; Maria C. Banatao Trust (the &amp;quot;Trust&amp;quot;). The contracts provide that the Trust will deliver a certain number of shares at the end of the contract depending on the Issuer's common stock price on such date.  1) a contract for 250,000 shares was entered into on  03/07/02 with a settlement date on 03/05/04; 2) a contract for 150,000 shares was entered into on 03/12/02 with a settlement date on 03/12/04; 3) a contract for 100,000 shares was entered into on 04/01/02 with a settlement date on 03/12/04; 4)  a contract for 300,000 shares was entered into on 06/28/02 with a settelement date on 06/28/05; 5) a contract for 100,000 shares was entered into on 09/10/02 with a settlement date on 09/09/05; 6) a contract for 100,000 shares was entered into on 09/23/02 with a settlement date on 09/23/05.</footnote><footnote id="F6">Continuation of Footnote #5 :  Exact pricing terms for each contract are determined on the respective settlement dates in accordance with the applicable contract.</footnote></footnotes><ownerSignature><signatureName>Matthew Gloss, by Power of Attorney</signatureName><signatureDate>2003-06-10</signatureDate></ownerSignature></ownershipDocument>

