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

    <documentType>4</documentType><periodOfReport>2003-07-01</periodOfReport>
    <issuer>
        <issuerCik>0001058057</issuerCik>
        <issuerName>MARVELL TECHNOLOGY GROUP LTD</issuerName>
        <issuerTradingSymbol>MRVL</issuerTradingSymbol></issuer><reportingOwner><reportingOwnerId>
            <rptOwnerCik>0001192171</rptOwnerCik>
            <rptOwnerName>CHANG KUO WEI HERBERT</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>2003-07-01</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>S</transactionCode><equitySwapInvolved>0</equitySwapInvolved></transactionCoding><transactionAmounts><transactionShares><value>160900</value></transactionShares><transactionValue><value>35.22</value></transactionValue><transactionAcquiredDisposedCode><value>D</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>35000</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>I</value></directOrIndirectOwnership><natureOfOwnership><value>By Investar Burgeon Venture Capital, Inc.</value></natureOfOwnership></ownershipNature></nonDerivativeSecurity><nonDerivativeSecurity><securityTitle><value>Common Stock</value></securityTitle><transactionDate><value>2003-07-01</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>J</transactionCode><equitySwapInvolved>0</equitySwapInvolved><footnoteId id="F2"/></transactionCoding><transactionAmounts><transactionShares><value>189665</value></transactionShares><transactionValue><value>0</value><footnoteId id="F2"/></transactionValue><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>370665</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>D</value></directOrIndirectOwnership><natureOfOwnership><value></value></natureOfOwnership></ownershipNature></nonDerivativeSecurity><nonDerivativeSecurity><securityTitle><value>Common Stock</value></securityTitle><transactionDate><value>2003-07-01</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>J</transactionCode><equitySwapInvolved>0</equitySwapInvolved><footnoteId id="F2"/></transactionCoding><transactionAmounts><transactionShares><value>298854</value></transactionShares><transactionValue><value>0</value><footnoteId id="F2"/></transactionValue><transactionAcquiredDisposedCode><value>D</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>85286</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>I</value></directOrIndirectOwnership><natureOfOwnership><value>Investar Capital Inc.</value></natureOfOwnership></ownershipNature></nonDerivativeSecurity><nonDerivativeSecurity><securityTitle><value>Common Stock</value></securityTitle><transactionDate><value>2003-07-01</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>J</transactionCode><equitySwapInvolved>0</equitySwapInvolved><footnoteId id="F3"/></transactionCoding><transactionAmounts><transactionShares><value>23850</value></transactionShares><transactionValue><value>0</value><footnoteId id="F3"/></transactionValue><transactionAcquiredDisposedCode><value>D</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>0</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>I</value></directOrIndirectOwnership><natureOfOwnership><value>Investar Avantgarde Holding, Inc.</value></natureOfOwnership></ownershipNature></nonDerivativeSecurity><nonDerivativeSecurity><securityTitle><value>Common Stock</value></securityTitle><transactionDate><value>2003-07-01</value></transactionDate><transactionCoding><transactionFormType>4</transactionFormType><transactionCode>J</transactionCode><equitySwapInvolved>0</equitySwapInvolved><footnoteId id="F3"/></transactionCoding><transactionAmounts><transactionShares><value>6814</value></transactionShares><transactionValue><value>0</value><footnoteId id="F3"/></transactionValue><transactionAcquiredDisposedCode><value>A</value></transactionAcquiredDisposedCode></transactionAmounts><postTransactionAmounts><sharesOwnedFollowingTransaction><value>6814</value></sharesOwnedFollowingTransaction></postTransactionAmounts><ownershipNature><directOrIndirectOwnership><value>I</value></directOrIndirectOwnership><natureOfOwnership><value>By Spouse</value><footnoteId id="F1"/><footnoteId id="F4"/></natureOfOwnership></ownershipNature></nonDerivativeSecurity><footnotes><footnote id="F1">The Reporting Person disclaims beneficial ownership of the shares held by his Spouse.</footnote><footnote id="F2">On July 1, 2003 Investar Capital Inc. effected a pro-rata distribution of its shares of the Issuer to certain affiliated parties, as noted herein, as well as to certain non-affiliated parties.  The receipt of shares by each such non-affiliated party are not reflected herein.  </footnote><footnote id="F3">On July 1, 2003 Investar Avantgarde Holding, Inc. effected a pro-rata distribution of its shares of the Issuer to certain affiliated parties, as noted herein, as well as to certain non-affiliated parties.  The receipt of shares by each such non-affiliated party are not reflected herein.  </footnote><footnote id="F4">Footnote for Table II - Summary of Derivative Securities: The Reporting Person owns directly three Stock Options (Right to Buy) with different vesting schedules described as follows: (i) 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; (ii) 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 (iii) 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.  All three 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></footnotes><ownerSignature><signatureName>Matthew Gloss, by Power of Attorney</signatureName><signatureDate>2003-07-03</signatureDate></ownerSignature></ownershipDocument>

